(269.) The propagation of motion through all substances, whether of a

single impulse, as a blow or thrust, or of one frequently and regularly

repeated, such as a jarring or vibratory movement, depends wholly on

these molecular forces; and it is on such propagation that sound and

very probably light depend. To conceive the manner in which a motion

may be conveyed from one part of a substance to another, whether solid

or fluid, we may attend to what takes place when a wave is made to

run along a stretched string, or the surface of still water. Every

part of the string, or water, is in succession moved from its place,

and agitated with a motion similar to that of the original impulse,

leaving its place and returning to it, and when one part ceases to move

the next receives as it were the impression, and forwards it onward.

This may seem a slow and circuitous process in description; but when

sound, for example, is conveyed through the air, we are to consider,

1st, that the air, the substance actually in motion, is extremely light

and acted upon by a very powerful elasticity, so that the force which

propagates the motion, or by which the particles adjacent act on, and

urge forward, each other, is very great, compared with the quantity of

materials set in motion by it: and the same is true, even in a greater

degree, in liquids and solids; for in these the elastic forces are even

greater, in proportion to the weight, than in air.

(270.) A general notion of the mode in which sounds are conveyed

through the air was not altogether deficient among the ancients; but

it is to Newton that we owe the first attempt to analyze the process,

and show correctly what takes place in the communication of motion

from particle to particle. Reasoning on the properties of the air as

an elastic body, he showed the effect of an impulse on any portion of

it to consist in a condensation of the air immediately adjacent in the

direction of the impulse, which then, re-acting by its spring, drives

back the portion which had advanced to its original place, and at the

same time urges forward the portion before it, in the direction of the

impulse, so that every particle alternately advances and retreats.

But, in pursuing this idea into its details, Newton fell into some

errors which were pointed out by Cramer, though their origin was not

traced, nor the reasoning corrected, till the subject was resumed by

Lagrange and Euler; nor is this any impeachment of the penetration of

our immortal countryman. The mathematical theory of the propagation

of sound, and of vibratory and undulatory motions in general, is

one of the utmost intricacy; and, in spite of every exertion on the

part of the most expert geometers, continues to this day to give

continual occasion for fresh researches; while phenomena are constantly

presenting themselves, which show how far we are from being able to

deduce all the particulars, even of cases comparatively simple, by any

direct reasoning from first principles.

(271.) Whenever an impulse of any kind is conveyed by the air, to our

ears, it produces the impression of sound; but when such an impulse

is regularly and uniformly repeated in extremely rapid succession, it

gives us that of a musical note, the pitch of the note depending on

the rapidity of the succession (see art. 153.). The sense of harmony,

too, depends on the periodical recurrence of coincident impulses on the

ear, and affords, perhaps, the only instance of a sensation for whose

pleasing impression a distinct and intelligible reason can be assigned.

(272.) Acoustics, then, or the science of sound, is a very considerable

branch of physics, and one which has been cultivated from the earliest

ages. Even Pythagoras and Aristotle were not ignorant of the general

mode of its transmission through the air, and of the nature of harmony;

but as a branch of science, independent of its delightful application

in the art of music, it could be hardly said to exist, till its nature

and laws became a matter of experimental enquiry to Bacon and Galileo,

Mersenne and Wallis; and of mathematical investigation to Newton, and

his illustrious successors, Lagrange and Euler. From that time its

progress, as a branch both of mathematical and experimental science,

has been constant and accelerated. A curious and beautiful method of

observation, due to Chladni, consists in the happy device of strewing

sand over the surfaces of bodies in a state of sonorous vibration,

and marking the figures it assumes. This has made their motions

susceptible of ocular examination, and has been lately much improved

on, and varied in its application, by M. Savart, to whom we also owe a

succession of instructive researches on every point connected with the

subject of sound, which may rank among the finest specimens of modern

experimental enquiry. But the subject is far from being exhausted; and,

indeed, there are few branches of physics which promise at once so much

amusing interest, and such important consequences, in its bearings on

other subjects, and especially, through the medium of strong analogies,

on that of light.

_Light and Vision._

(273.) The nature of light has always been involved in considerable

doubt and mystery. The ancients could scarcely be said to have any

opinion on the subject, unless, indeed, it could be considered such

to affirm that distant bodies could not be put into communication

without an intermedium; and that, therefore, there must be _something_

between the eye and the thing seen. What that something is, however,

they could only form crude and vague conjectures. One supposed that

the eyes themselves emit rays or emanations of some unknown kind, by

which distant objects are as it were felt; a singularly unfortunate

idea, since it gives no reason why objects should not be equally

well seen in the dark--no account, in short, of the part performed

by _light_ in vision. Others imagined that all visible objects are

constantly throwing out from them, in all directions, some sort of

resemblances or spectral forms of themselves, which, when received by

the eyes, produce an impression of the objects. Vague and clumsy as

this hypothesis obviously is, it assigns to the object a power, and to

light a diffusive propagation in all directions, which are, the one and

the other, independent of our eyes, and therefore goes to separate the

phenomena of _light_ from those of _vision_.

(274.) The hypothesis of Newton is a refinement and improvement on

this idea. Instead of spectra or resemblances, he supposes luminous

objects actually to dart out from them in all directions, particles,

of inconceivable minuteness (as indeed they must be, having such

an enormous velocity (see 17.), not to dash in pieces every thing

they strike upon). These particles he supposes to be acted upon by

attractive and repulsive forces, residing in all material bodies, the

latter extending to some very small distance beyond their surfaces; and

by the action of these forces to be turned aside from their natural

straight-lined course, without ever coming in actual contact with the

particles themselves of the bodies on which they fall, but either being

turned back and _reflected_ by the repulsive forces before they reach

them, or penetrating between their intervals, as a bird may be supposed

to fly through the branches of a forest, and undergoing all their

actions, to take at quitting them a direction finally determined by

the position of the surface at which they emerge with respect to their

course.

(275.) This hypothesis, which was discussed and reasoned upon by Newton

in a manner worthy of himself, affords, by the application of the

same dynamical laws which he had applied with so much success to the

explanation of the planetary motions, not merely a plausible, but a

perfectly reasonable and fair explanation of all the _usual_ phenomena

of light known in his time. His own beautiful discoveries, too, of

the different refrangibilities of the differently coloured rays,

were perfectly well represented in this theory, by simply admitting

a difference of velocity in the particles, which produce in the eye

the sensations of different colours. And had the properties of light

remained confined to these, there would have been no occasion to have

resorted to any other mode of conceiving it.

(276.) A very different hypothesis had, however, been suggested about

the same period by Huyghens, who supposed light to be produced in the

same manner with sound, by the communication of a vibratory motion

from the luminous body to a highly elastic fluid, which he imagined

as filling all space, and as being less condensed within the limits

of space occupied by matter, and that to a greater or less extent,

according to the nature of the occupying substance. Thus, in place of

any thing actually thrown off, he substituted waves, or vibrations,

propagated in all directions from luminous bodies, through this medium,

or ether, as he called it. Huyghens, being himself a consummate

mathematician, was enabled to trace many of the consequences of this

hypothesis, and to show that the ordinary laws of reflection and

refraction were represented or accounted for by it, as well as by

Newton’s. But the hypothesis of Huyghens has not been fully successful

in accounting for what may be considered the chief of all optical

facts, the production of colours in the ordinary refraction of

light by a prism, of which the theory of Newton gives a complete and

elegant explanation; and the discovery of which by him marks one of

the greatest epochs in the annals of experimental science. This, which

has been often urged in objection to it, remains still, if not quite

unanswered, at least only imperfectly removed.

(277.) Other phenomena, however, were not wanting to afford a further

trial of the _explanatory powers_ of either hypothesis. The diffraction

or inflection of light, discovered by Grimaldi, a Jesuit of Bologna,

seemed to indicate that the rays of light were turned aside from their

straight course by merely passing near bodies of every description.

These phenomena, which are very curious and beautiful, were minutely

examined by Newton, and referred by him to the action of repulsive

forces extending to a sensible distance from the surfaces of bodies;

and his explanation, so far as the facts known to him are concerned,

appears as satisfactory as could reasonably be then expected; and much

more so than any thing which could at that time be produced on the side

of the hypothesis of Huyghens, which, in fact, seemed incapable of

giving any account whatever of them.

(278.) Another class of delicate and splendid optical phenomena, which

had begun to attract attention somewhat previous to Newton’s time,

seemed to leave both hypotheses equally at a loss. These were the

colours exhibited by very thin films, either of a liquid (such as a

soap-bubble), or of air, as when two glasses are laid together with

only air between them. These colours were examined by Newton with a

minuteness and care altogether unexampled in experimental philosophy

at that time, and with which few researches undertaken since will bear

to stand in competition. Their result was a theory of a very singular

nature, which he grounded on an hypothesis of what he termed _fits of

easy transmission and reflection_; and which supposed each ray of light

to pass in its progress periodically through a succession of states

such as would alternately dispose it to penetrate or be reflected back

from the surface of a body on which it might fall. The simplest way

in which the reader may conceive this hypothesis, is to regard every

particle of light as a sort of little magnet revolving rapidly about

its own centre while it advances in its course, and thus alternately

presenting its attractive and repulsive pole, so that when it arrives

at the surface of a body with its repulsive pole foremost, it is

repelled and reflected; and when the contrary, attracted, so as to

enter the surface. Newton, however, very cautiously avoided announcing

his theory in this or any similar form, confining himself entirely to

general language. In consequence, it has been confidently asserted

by all his followers, that the doctrine of fits of easy reflection

and transmission, as laid down by him, is substantially nothing more

than a statement of facts. Were it so, it is clear that any other

theory which should offer a just account of the same phenomena must

ultimately involve and coincide with that of Newton. But this, as we

shall presently see, is not the case; and this instance ought to serve

to make us extremely cautious how we employ, in stating physical laws

derived from experiment, language which involves any thing in the

slightest degree theoretical, if we would present the laws themselves

in a form which no future research shall modify or subvert.

(279.) A third class of optical phenomena, which were likewise

discovered while Newton was yet engaged in his optical researches, was

that exhibited by doubly refracting crystals. In what the phenomenon of

double refraction consists, we have already had occasion to explain.

The fact itself was first noticed by Erasmus Bartolin in the crystal

called Iceland spar; and was studied with attention by Huyghens, who

ascertained its laws, and referred it with remarkable ingenuity and

success to his theory of light, by the additional hypothesis of such

a constitution of his ethereal medium within the crystal as should

enable it to convey an impulse faster in one direction than another:

as if, for example’s sake, we should suppose a sound conveyed through

the air with different degrees of rapidity in a vertical and horizontal

direction.

(280.) Some remarkable facts accompanying the double refraction

produced by Iceland spar, which Bartolin, Huyghens, and Newton, had

observed, led the latter to conceive the singular idea that a ray of

light after its emergence from such a crystal acquires _sides_, that

is to say, distinct relations to surrounding space, which it carries

with it through its whole subsequent course, and which give rise to all

those curious and complicated phenomena which are now known under the

name of the _polarization of light_. These results, however, appeared

so extraordinary, and offered so little handle for further enquiry,

that their examination dropped, as if by common consent; Newton himself

resting content with urging strongly the apparent incompatibility of

these properties with the Huyghenian doctrine, but without making any

attempt to explain them by his own.

(281.) From the period of Newton’s optical discoveries to the

commencement of the present century, no great accession to our

knowledge of the nature of light was made, if we except one, which,

from its invaluable practical application, must ever hold a prominent

place in the annals both of art and science: we mean, the discovery

of the principle of the achromatic telescope, which originated in a

discussion between the celebrated geometer Euler, Klingenstierna, an

eminent Swedish philosopher, and our own countryman, the admirable

optician Dollond, on the occasion of certain abstract theoretical

investigations of the former, which led him to speculate on its

_possibility_, and which ultimately terminated in its complete and

happy _execution_ by the latter; a memorable case in science, though

not a singular one, where the speculative geometer in his chamber,

apart from the world, and existing among abstractions, has originated

views of the noblest practical application.[49]

(282.) The explanation which our knowledge of optical laws affords of

the mechanism of the eye, and the process by which vision is performed,

is as complete and satisfactory as that of hearing by the propagation

of motion through the air. The camera obscura, invented by Baptista

Porta in 1560, gave the first idea how the actual images of external

objects might be conveyed into the eye, but it was not till after a

considerable interval that Kepler, the immortal discoverer of those

great laws which regulate the periods and motions of the planets,

pointed out distinctly the offices performed by the several parts

of the eye in the act of vision. From this to the invention of the

telescope and microscope there would seem but a small step, but it is

to accident rather than design that it is due; and its re-invention

by Galileo, on a mere description of its effects, may serve, among

a thousand similar instances, to show that inestimable practical

applications lie open to us, if we can only once bring ourselves

to conceive their possibility, a lesson which the invention of the

achromatic telescope itself, as we have above related it, not less

strongly exemplifies.

(283.) The little instrument with which Galileo’s splendid discoveries

were made was hardly superior in power to an ordinary finder of the

present day; but it was rapidly improved on, and in the hands of

Huyghens attained to gigantic dimensions and very great power. It was

to obviate the necessity of the enormous length required for these

telescopes, and yet secure the same power, that Gregory and Newton

devised the reflecting telescope, which has since become a much

more powerful instrument than its original inventors probably ever

contemplated.

(284.) The telescope, as it exists at present, with the improvements

in its structure and execution which modern artists have effected, must

assuredly be ranked among the highest and most refined productions of

human art; that in which man has been able to approximate most closely

to the workmanship of nature, and which has conferred upon him, if

not another sense, at least an exaltation of one already possessed

by him that merits almost to be regarded as a new one. Nor does it

appear yet to have reached its ultimate perfection, to which indeed

it is difficult to assign any bounds, when we take into consideration

the wonderful progress which workmanship of every kind is making,

and the delicacy, far superior to that of former times, with which

materials may now be wrought, as well as the ingenious inventions and

combinations which every year is bringing forth for accomplishing the

same ends by means hitherto unattempted.[50]

(285.) After a long torpor, the knowledge of the properties of light

began to make fresh progress about the end of the last century,

advancing with an accelerated rapidity, which has continued unabated

to the present time. The example was set by our late admirable and

lamented countryman, Dr. Wollaston, who re-examined and verified

the laws of double refraction in Iceland spar announced by Huyghens.

Attention being thus drawn to the subject, the geometry of Laplace

soon found a means of explaining at least one portion of the mystery

of this singular phenomenon, by the Newtonian theory of light, applied

under certain supposed conditions; and the reasoning which led him to

the result (at that time quite unexpected), may justly be regarded as

one of his happiest efforts. The prosecution of the subject, which had

now acquired a high degree of interest, was encouraged by the offer

of a prize on the part of the French Academy of Sciences; and it was

in a memoir which received this honourable reward on that occasion,

in 1810, that Malus, a retired officer of engineers in the French

army, announced the great discovery of the _polarization of light_ by

ordinary reflection at the surface of a transparent body.

(286.) Malus found that when a beam of light is reflected from the

surface of such a body at a certain angle, it acquires precisely the

same singular property which is impressed upon it in the act of double

refraction, and which Newton had before expressed by saying that it

possessed _sides_. This was the first circumstance which pointed out a

connection between that hitherto mysterious phenomenon and any of the

ordinary modifications of light; and it proved ultimately the means of

bringing the whole within the limits, if not of a complete explanation,

at least of a highly plausible theoretical representation. So true is,

in science, the remark of Bacon, that no natural phenomenon can be

adequately studied _in itself alone_, but, to be understood, must be

considered _as it stands connected with all nature_.

(287.) The new class of phenomena thus disclosed were immediately

studied with diligence and success, both abroad by Malus and Arago, and

at home by our countryman Dr. Brewster, and their laws investigated

with a care proportioned to their importance; when another and

apparently still more extraordinary class of phenomena presented

itself in the production of the most vivid and beautiful colours

(every way resembling those observed by Newton in thin films of air

or liquids, only infinitely more developed and striking,) in certain

transparent crystallized substances, when divided into flat plates in

particular directions, and exposed in a beam of polarized light. The

attentive examination of these colours by Wollaston, Biot, and Arago,

but more especially by Brewster, speedily led to the disclosure of a

series of optical phenomena so various, so brilliant, and evidently

so closely connected with the most important points relating to the

intimate structure of crystallized bodies, as to excite the highest

interest,--that sort of interest which is raised when we feel we are

on the eve of some extraordinary discovery, and expect every moment

that some leading fact will turn up, which will throw light on all that

appears obscure, and reduce into order all that seems anomalous.

(288.) This expectation was not disappointed. So long before the time

we are speaking of as the first year of the present century, our

illustrious countryman, the late Dr. Thomas Young, had established a

principle in optics, which, regarded as a physical law, has hardly its

equal for beauty, simplicity, and extent of application, in the whole

circle of science. Considering the manner in which the vibrations of

two musical sounds arriving at once at the ear affect the sense with

an impression of sound or silence according as they conspire or oppose

each other’s effects, he was led to the idea that the same ought to

hold good with light as with sound, if the theory which makes light

analogous to sound be the true one; and that, therefore, two rays of

light, setting off from the same origin, at the same instant, and

arriving at the same place by different routes, ought to strengthen

or wholly or partially destroy each other’s effects according to

the difference in length of the routes described by them. That two

lights should in any circumstances combine to produce darkness may

be considered strange, but is _literally true_; and it had even been

noticed long ago as a singular and unaccountable fact by Grimaldi, in

his experiments on the inflection of light. The experimental means by

which Dr. Young confirmed this principle, which is known in optics

by the name of the _interference_ of the rays of light, were as

simple and satisfactory as the principle itself is beautiful; but the

verifications of it, drawn from the explanation it affords of phenomena

apparently the most remote, are still more so. Newton’s colours of thin

films were the first phenomena to which its author applied it with full

success. Its next remarkable application was to those of diffraction,

of which, in the hands of M. Fresnel, a late eminent French geometer,

it also furnished a complete explanation, and that, too, in cases to

which Newton’s hypothesis could not apparently be made to apply, and

through a complication of circumstances which might afford a very

severe test of any hypothesis.

(289.) A simple and beautiful experiment on the interferences of

polarized light due to Fresnel and Arago enabled them to bring Dr.

Young’s law to bear on the colours produced by crystallized plates in a

polarized beam, and by so doing afforded a key to all the intricacies

of these magnificent but complex phenomena. Nothing now was wanting to

a rational theory of double refraction but to frame an hypothesis of

some mode in which light might be conceived to be propagated through

the elastic medium supposed to convey it in such a way as not to be

contradictory to any of the facts, nor to the general laws of dynamics.

This essential idea, without which every thing that had been before

done would have been incomplete, was also furnished by Dr. Young, who,

with a sagacity which would have done honour to Newton himself, had

declared, that to accommodate the doctrine of Huyghens to the phenomena

of polarized light it is necessary to conceive the mode of propagation

of a luminous impulse through the ether, differently from that of a

sonorous one through the air. In the latter, the particles of the air

_advance_ and _recede_; in the former, those of the ether must be

supposed to _tremble laterally_.

(290.) Taking this as the groundwork of his reasoning, Fresnel

succeeded in erecting on it a theory of polarization and double

refraction, so happy in its adaptation to facts, and in the coincidence

with experience of results deduced from it by the most intricate

analysis, that it is difficult to conceive it unfounded. If it be

so, it is at least the most curiously artificial system that science

has yet witnessed; and whether it be so or not, so long as it serves

to group together in one comprehensive point of view a mass of facts

almost infinite in number and variety, to reason from one to another,

and to establish analogies and relations between them; on whatever

hypothesis it may be founded, or whatever arbitrary assumptions it

may make respecting structures and modes of action, it can never be

regarded as other than a most real and important accession to our

knowledge.

(291.) Still, it is by no means impossible that the Newtonian theory of

light, if cultivated with equal diligence with the Huyghenian, might

lead to an equally plausible explanation of phenomena now regarded

as beyond its reach. M. Biot is the author of the hypothesis we have

already mentioned of a rotatory motion of the particles of light about

their axes. He has employed it only for a very limited purpose; but

it might doubtless be carried much farther; and by admitting only the

regular emission of the luminous particles at equal intervals of time,

and in similar states of motion from the shining body, which does not

seem a very forced supposition, all the phenomena of interference at

least would be readily enough explained without the admission of an

ether.

(292.) The optical examination of crystallized substances affords

one among many fine examples of the elucidation which every branch

of science is capable of affording to every other. The indefatigable

researches of Dr. Brewster and others have shown that the phenomena

exhibited by polarized light in its transmission through crystals

afford a certain indication of the most important points relating to

the structure of the crystals themselves, and thus become most valuable

characters by which to recognise their internal constitution. It was

Newton who first showed of what importance as a physical character,--as

the indication of other properties,--the action of a body on light

might become; but the characters afforded by the use of polarized light

as an instrument of experimental enquiry are so marked and intimate,

that they may almost be said to have furnished us with a kind of

intellectual sense, by which we are enabled to scrutinize the internal

arrangement of those wonderful structures which Nature builds up by

her refined and invisible architecture, with a delicacy eluding our

conception, yet with a symmetry and beauty which we are never weary

of admiring. In this point of view the science of optics has rendered

to mineralogy and crystallography services not less important than to

astronomy by the invention of the telescope, or to natural history by

that of the microscope; while the relations which have been discovered

to exist between the optical properties of bodies and their crystalline

forms, and even their chemical habitudes, have afforded numerous

and beautiful instances of general laws concluded from laborious and

painful induction, and curiously exemplifying the simplicity of nature

as it emerges slowly from an entangled mass of particulars in which, at

first, neither order nor connection can be traced.

CHAP. III.

OF COSMICAL PHENOMENA.

_Astronomy and Celestial Mechanics._

(293.) Astronomy, as has been observed in the former part of this

discourse, as a science of observation, had made considerable progress

among the ancients: indeed, it was the only branch of physical science

which could be regarded as having been cultivated by them with any

degree of assiduity or real success. The Chaldean and Egyptian records

had furnished materials from which the motions of the sun and moon

could be calculated with sufficient exactness for the prediction of

eclipses; and some remarkable cycles, or periods of years in which

the lunar eclipses return in very nearly the same order, had been

ascertained by observation. Considering the extreme imperfection of

their means of measuring time and space, this was, perhaps, as much

as could have been expected at that early period, and it was followed

up for a while in a philosophical spirit of just speculation, which,

if continued, could hardly have failed to lead to sound and important

conclusions.

(294.) Unfortunately, however, the philosophy of Aristotle laid it

down as a principle, that the celestial motions were regulated by laws

proper to themselves, and bearing no affinity to those which prevail

on earth. By thus drawing a broad and impassable line of separation

between celestial and terrestrial mechanics, it placed the former

altogether out of the pale of experimental research, while it at the

same time impeded the progress of the latter by the assumption of

principles respecting natural and unnatural motions, hastily adopted

from the most superficial and cursory remark, undeserving even the

name of observation. Astronomy, therefore, continued for ages a

science of mere record, in which theory had no part, except in so

far as it attempted to conciliate the inequalities of the celestial

motions with that assumed law of uniform circular revolution which

was alone considered consistent with the perfection of the heavenly

mechanism. Hence arose an unwieldy, if not self-contradictory, mass

of hypothetical motions of sun, moon, and planets, in circles, whose

centres were carried round in other circles, and these again in others

without end,--“cycle on epicycle, orb on orb,”--till at length, as

observation grew more exact, and fresh epicycles were continually

added, the absurdity of so cumbrous a mechanism became too palpable to

be borne. Doubts were expressed, to which the sarcasm of a monarch[51]

gave a currency they might not have obtained in a period when men

scarcely dared trust themselves to think; and at length Copernicus,

promulgating his own, or reviving the Pythagorean doctrine, which

places the sun in the centre of our system, gave to astronomy a

simplicity which, contrasted with the complication of the preceding

views, at once commanded assent.

(295.) An elegant writer[52], whom we have before had occasion to

quote, has briefly and neatly accounted for the confused notions which

so long prevailed respecting the constitution of our system, and the

difficulty experienced in acquiring a true notion of the disposition

of its parts. “We see it,” he observes, “not in _plan_, but in

_section_.” The reason of this is, that our point of observation lies

in its general plane, but the notion we aim at forming of it is not

that of its section, but of its plan. This is as if we should attempt

to read a book, or make out the countries on a map, with the eye on a

level with the paper. We can only judge directly of the distances of

objects by their sizes, or rather of their change of distance by their

change of size; neither have we any means of ascertaining, otherwise

than indirectly, even their positions, one among the other, from their

apparent places as seen by us. Now, the variations in apparent size

of the sun and moon are too small to admit of exact measure without

the use of the telescope, and the bodies of the planets cannot even be

distinguished as having any distinct size with the naked eye.

(296.) The Copernican system once admitted, however, this difficulty

of conception, at least, is effectually got over, and it becomes

a mere problem of geometry and calculation to determine, from the

observed places of a planet, its real orbit about the sun, and the

other circumstances of its motion. This Kepler accomplished for the

orbit of Mars, which he ascertained to be an ellipse having the sun

in one of its foci; and the same law, being extended by inductive

analogy to all the planets, was found to be verified in the case of

each. This with the other remarkable laws which are usually cited in

physical astronomy by the name of Kepler’s laws, constitute undoubtedly

the most important and beautiful system of geometrical relations which

have ever been discovered by a mere inductive process, independent of

any consideration of a theoretical kind. They comprise within them a

compendium of the motions of all the planets, and enable us to assign

their places in their orbits at any instant of time past or to come

(disregarding their mutual perturbations), provided certain purely

geometrical problems can be numerically resolved.

(297.) It was not, however, till long after Kepler’s time that the

real importance of these laws could be felt. Regarded in themselves,

they offered, it is true, a fine example of regular and harmonious

disposition in the greatest of all the works of creation, and a

striking contrast to the cumbersome mechanism of the cycles and

epicycles which preceded them; but there their utility seemed to

terminate, and, indeed, Kepler was reproached, and not without a

semblance of reason, with having rendered the actual calculation of

the places of the planets more difficult than before, the resources of

geometry being then inadequate to resolve the problems to which the

strict application of his laws gave rise.

(298.) The first result of the invention of the telescope and its

application to astronomical purposes, by Galileo, was the discovery

of Jupiter’s disc and satellites,--of a system offering a beautiful

miniature of that greater one of which it forms a portion, and

presenting to the eye of sense, at a single glance, that disposition of

parts which in the planetary system itself is discerned only by the eye

of reason and imagination (see 195.). Kepler had the satisfaction of

seeing it ascertained, that the law which he had discovered to connect

the times of revolution of the planets with their distances from the

sun, holds good also when applied to the periods of circulation of

these little attendants round the centre of their principal; thus

demonstrating it to be something more than a mere empirical rule, and

to depend on the intimate nature of planetary motion itself.

(299.) It had been objected to the doctrine of Copernicus, that, were

it true, Venus should appear sometimes horned like the moon. To this

he answered by admitting the conclusion, and averring that, should we

ever be able to see its actual shape, it _would_ appear so. It is easy

to imagine with what force the application would strike every mind when

the telescope confirmed this prediction, and showed the planet just as

both the philosopher and his objectors had agreed it ought to appear.

The history of science affords perhaps only one instance analogous to

this. When Dr. Hutton expounded his theory of the consolidation of

rocks by the application of heat, at a great depth below the bed of

the ocean, and especially of that of marble by actual fusion; it was

objected that, whatever might be the case with others, with calcareous

or marble rocks, at least, it was impossible to grant such a cause

of consolidation, since heat decomposes their substance and converts

it into quicklime, by driving off the carbonic acid, and leaving a

substance perfectly infusible, and incapable even of agglutination

by heat. To this he replied, that the pressure under which the heat

was applied would prevent the escape of the carbonic acid; and that

being retained, it might be expected to give that fusibility to the

compound which the simple quicklime wanted. The next generation saw

this anticipation converted into an observed fact, and verified by the

direct experiments of Sir James Hall, who actually succeeded in melting

marble, by retaining its carbonic acid under violent pressure.

(300.) Kepler, among a number of vague and even wild speculations on

the causes of the motions whose laws he had developed so beautifully

and with so much patient labour, had obtained a glimpse of the general

law of the inertia of matter, as applicable to the great masses of the

heavenly bodies as well as to those with which we are conversant on

the earth. After Kepler, Galileo, while he gave the finishing blow to

the Aristotelian dogmas which erected a barrier between the laws of

celestial and terrestrial motion, by his powerful argument and caustic

ridicule, contributed, by his investigations of the laws of falling

bodies and the motions of projectiles, to lay the foundation of a

true system of dynamics, by which motions could be determined from a

knowledge of the forces producing them, and forces from the motions

they produce. Hooke went yet farther, and obtained a view so distinct

of the mode in which the planets might be retained in their orbits

by the sun’s attraction, that, had his mathematical attainments been

equal to his philosophical acumen, and his scientific pursuits been

less various and desultory, it can hardly be doubted that he would have

arrived at a knowledge of the law of gravitation.

(301.) But every thing which had been done towards this great end,

before Newton, could only be regarded as smoothing some first

obstacles, and preparing a state of knowledge, in which powers like his

could be effectually exerted. His wonderful combination of mathematical

skill with physical research enabled him to invent, at pleasure, new

and unheard-of methods of investigating the effects of those causes

which his clear and penetrating mind detected in operation. Whatever

department of science he touched, he may be said to have formed afresh.

Ascending by a series of close-compacted inductive arguments to the

highest axioms of dynamical science, he succeeded in applying them

to the complete explanation of all the great astronomical phenomena,

and many of the minuter and more enigmatical ones. In doing this, he

had every thing to create: the mathematics of his age proved totally

inadequate to grapple with the numerous difficulties which were to

be overcome; but this, so far from discouraging him, served only to

afford new opportunities for the exertion of his genius, which, in the

invention of the method of fluxions, or, as it is now more generally

called, the differential calculus, has supplied a means of discovery,

bearing the same proportion to the methods previously in use, that

the steam-engine does to the mechanical powers employed before its

invention. Of the optical discoveries of Newton we have already spoken;

and if the magnitude of the objects of his astronomical discoveries

excite our admiration of the mental powers which could so familiarly

grasp them, the minuteness of the researches into which he there set

the first example of entering, is no less calculated to produce a

corresponding impression. Whichever way we turn our view, we find

ourselves compelled to bow before his genius, and to assign to the

name of NEWTON a place in our veneration which belongs to no other in

the annals of science. His era marks the accomplished maturity of the

human reason as applied to such objects. Every thing which went before

might be more properly compared to the first imperfect attempts of

childhood, or the essays of inexpert, though promising, adolescence.

Whatever has been since performed, however great in itself, and worthy

of so splendid and auspicious a beginning, has never, in point of

intellectual effort, surpassed that astonishing one which produced the

Principia.

(302.) In this great work, Newton shows all the celestial motions known

in his time to be consequences of the simple law, that every particle

of matter attracts every other particle in the universe with a force

proportional to the product of their masses directly, and the square

of their mutual distance inversely, and is itself attracted with an

equal force. Setting out from this, he explains how an attraction

arises between the great spherical masses of which our system consists,

regulated by a law precisely similar in its expression; how the

elliptic motions of planets about the sun, and of satellites about

their primaries, according to the exact rules inductively arrived at

by Kepler, result as necessary consequences from the same general law

of force; and how the orbits of comets themselves are only particular

cases of planetary movements. Thence proceeding to applications of

greater difficulty, he explains how the perplexing inequalities of

the moon’s motion result from the sun’s disturbing action; how tides

arise from the unequal attraction of the sun as well as of the moon

on the earth, and the ocean which surrounds it; and, lastly, how the

precession of the equinoxes is a necessary consequence of the very same

law.

(303.) The immediate successors of Newton found full occupation

in verifying his discoveries, and in extending and improving the

mathematical methods which it had now become manifest were to prove

the keys to an inexhaustible treasure of knowledge. The simultaneous

but independent discovery of a method of mathematical investigation in

every respect similar to that of Newton, by Leibnitz, while it created

a degree of national jealousy which can now only be regretted, had the

effect of stimulating the continental geometers to its cultivation, and

impressing on it a character more entirely independent of the ancient

geometry, to which Newton was peculiarly attached. It was fortunate

for science that it did so; for it was speedily found that (with one

fine exception on the part of our countryman Maclaurin, followed up,

after a long interval, by the late Professor Robison of Edinburgh, with

equal elegance,) the geometry of Newton was like the bow of Ulysses,

which none but its master could bend; and that, to render his methods

available beyond the points to which he himself carried them, it was

necessary to strip them of every vestige of that antique dress in which

he had delighted to clothe them. This, however, the countrymen of

Newton were very unwilling to do; and they paid the penalty in finding

themselves condemned to the situation of lookers on, while their

continental neighbours both in Germany and France were pushing forward

in the career of mathematico-physical discovery with emulous rapidity.

(304.) The legacy of research which Newton may be said to have left

to his successors was truly immense. To pursue, through all its

intricacies, the consequences of the law of gravitation; to account for

all the inequalities of the planetary movements, and the infinitely

more complicated, and to us more important ones, of the moon; and to

give, what Newton himself certainly never entertained a conception

of, a demonstration of the stability and permanence of the system,

under all the accumulating influence of its internal perturbations;

this labour, and this triumph, were reserved for the succeeding

age, and have been shared in succession by Clairaut, D’Alembert,

Euler, Lagrange and Laplace. Yet so extensive is the subject, and so

difficult and intricate the purely mathematical enquiries to which it

leads, that another century may yet be required to go through with

the task. The recent discoveries of astronomers have supplied matter

for investigation, to the geometers of this and the next generation,

of a difficulty far surpassing any thing that had before occurred.

Five primary planets have been added to our system; four of them

since the commencement of the present century, and these, singularly

deviating from the general analogy of the others, and offering _cases

of difficulty_ in theory, which no one had before contemplated. Yet

even the intricate questions to which these bodies have given rise

seem likely to be surpassed by those which have come into view, with

the discovery of several comets revolving in elliptic orbits, like the

planets, round the sun, in very moderate periods. But the resources of

modern geometry seem, so far from being exhausted, to increase with the

difficulties they have to encounter, and already, among the successors

of Lagrange and Laplace, the present generation has to enumerate a

powerful array of names, which promise to render it not less celebrated

in the annals of physico-mathematical research than that which has just

passed away.

(305.) Meanwhile the positions, figures, and dimensions of all the

planetary orbits, are now well known, and their variations from century

to century in great measure determined; and it has been generally

demonstrated, that all the changes which the mutual actions of the

planets on each other can produce in the course of indefinite ages,

are _periodical_, that is to say, increasing to a certain extent (and

that never a very great one), and then again decreasing; so that the

system can never be destroyed or subverted by the mutual action of its

parts, but keeps constantly oscillating, as it were, round a certain

mean state, from which it can never deviate to any ruinous extent. In

particular the researches of Laplace, Lagrange, and Poisson, have shown

the ultimate invariability of the mean distance of each planet from the

sun, and consequently of its periodic time. Relying on these grand

discoveries, we are enabled to look forward, from the point of time

which we now occupy, many thousands of years into futurity, and predict

the state of our system without fear of material error, but such as

may arise from causes whose existence at present we have no reason to

suppose, or from interference which we have no right to anticipate.

(306.) A correct enumeration and description of the fixed stars in

catalogues, and an exact knowledge of their position, supply the

only effectual means we can have of ascertaining what changes they

are liable to, and what motions, too slow to deprive them of their

usual epithet, _fixed_, yet sufficient to produce a sensible change

in the lapse of ages, may exist among them. Previous to the invention

of the compass, they served as guides to the navigator by night; but

for this purpose, a very moderate knowledge of a few of the principal

ones sufficed. Hipparchus was the first astronomer, who, excited

by the appearance of a new star, conceived the idea of forming a

catalogue of the stars, with a view to its use as an astronomical

record, “by which,” says Pliny, “posterity will be able to discover,

not only whether they are born and die, but also whether they change

their places, and whether they increase or decrease.” His catalogue,

containing more than 1000 stars, was constructed about 128 years before

Christ. It was in the course of the laborious discussion of his own and

former observations of them, undertaken with a view to the formation of

this catalogue, that he first recognised the fact of that slow, general

advance of all the stars eastward, when compared with the place of

the equinox, which is known under the name of the precession of the

equinoxes, and which Newton succeeded in referring to a motion in the

earth’s axis, produced by the attraction of the sun and moon.

(307.) Since Hipparchus, at various periods in the history of

astronomy, catalogues of stars have been formed, among which that

of Ulugh Begh, comprising about 1000 stars, constructed in 1437, is

remarkable as the production of a sovereign prince, working personally

in conjunction with his astronomers; and that of Tycho Brahe,

containing 777 stars, constructed in 1600, as having originated in a

phenomenon similar to that which drew the attention of Hipparchus. In

more recent times, astronomers provided with the finest instruments

their respective eras could supply, and established in observatories,

munificently endowed by the sovereigns and governments of different

European nations, have vied and are still vying with each other,

in extending the number of registered stars, and giving the utmost

possible degree of accuracy to the determination of their places.

Among these, it would be ungrateful not to claim especial notice

for the superb series of observations which, under a succession of

indefatigable and meritorious astronomers, has, for a very long period,

continued to emanate from our own national observatory of Greenwich.

(308.) The distance of the fixed stars is so immense, that every

attempt to assign a limit, _within which_ it _must_ fall, has hitherto

failed. The enquiries of astronomers of all ages have been directed

to ascertain this distance, by taking the dimensions of our own

particular system of sun and planets, or of the earth itself, as the

unit of a scale on which it might be measured. But although many have

imagined that their observations afforded grounds for the decision

of this interesting point, it has uniformly happened either that the

phenomena on which they relied have proved to be referable to other

causes not previously known, and which the superior accuracy of their

researches has for the first time brought to light; or to errors

arising from instrumental imperfections and unavoidable defects of the

observations themselves.

(309.) The only indication we can expect to obtain of the actual

distance of a star, would consist in an annual change in its apparent

place corresponding to the motion of the earth round the sun, called

its _annual parallax_, and which is nothing more than the measure

of the apparent size of the earth’s orbit as seen from the star.

Many observers have thought they have detected a measurable amount

of this parallax; but as astronomical instruments have advanced in

perfection, the quantity which they have successively assigned to it

has been continually reduced within narrower and narrower limits,

and has invariably been commensurate with the errors to which the

instruments used might fairly be considered liable. The conclusion this

strongly presses on us is, that it is really a quantity too small to

admit of distinct measurement in the present state of our means for

that purpose; and that, therefore, the distance of the stars must be

a magnitude of such an order as the imagination almost shrinks from

contemplating. But this increase in our scale of dimension calls for

a corresponding enlargement of conception in all other respects. The

same reasoning which places the stars at such immeasurable remoteness,

exalts them at the same time into glorious bodies, similar to, and even

far surpassing, our own sun, the centres perhaps of other planetary

systems, or fulfilling purposes of which we can have no idea, from any

analogy in what passes immediately around us.

(310.) The comparison of catalogues, published at different periods,

has given occasion to many curious remarks, respecting changes both

of place and brightness among the stars, to the discovery of variable

ones which lose and recover their lustre periodically, and to that of

the disappearance of several from the heavens so completely as to have

left no vestige discernible even by powerful telescopes. In proportion

as the construction of astronomical and optical instruments has gone on

improving, our knowledge of the contents of the heavens has undergone

a corresponding extension, and, at the same time, attained a degree of

precision which could not have been anticipated in former ages. The

places of all the principal stars in the northern hemisphere, and of a

great many in the southern, are now known to a degree of nicety which

must infallibly detect any real motions that may exist among them,

and has in fact done so, in a great many instances, some of them very

remarkable ones.

(311.) It is only since a comparatively recent date, however, that

any great attention has been bestowed on the smaller stars, among

which there can be no doubt of the most interesting and instructive

phenomena being sooner or later brought to light. The minute

examination of them with powerful telescopes, and with delicate

instruments for the determination of their places, has, indeed, already

produced immense catalogues and masses of observations, in which

thousands of stars invisible to the naked eye are registered; and has

led to the discovery of innumerable important and curious facts, and

disclosed the existence of whole classes of celestial objects, of a

nature so wonderful as to give room for unbounded speculation on the

extent and construction of the universe.

(312.) Among these, perhaps the most remarkable are the revolving

double stars, or stars which, to the naked eye or to inferior

telescopes, appear single; but, if examined with high magnifying

powers, are found to consist of two individuals placed almost close

together, and which, when carefully watched, are (many of them) found

to revolve in regular elliptic orbits about each other; and so far

as we have yet been able to ascertain, to obey the same laws which

regulate the planetary movements. There is nothing calculated to give a

grander idea of the scale on which the sidereal heavens are constructed

than these beautiful systems. When we see such magnificent bodies

united in pairs, undoubtedly by the same bond of mutual gravitation

which holds together our own system, and sweeping over their enormous

orbits, in periods comprehending many centuries, we admit at once that

they must be accomplishing ends in creation which will remain for ever

unknown to man; and that we have here attained a point in science

where the human intellect is compelled to acknowledge its weakness, and

to feel that no conception the wildest imagination can form will bear

the least comparison with the intrinsic greatness of the subject.

_Geology._

(313.) The researches of physical astronomy are confessedly incompetent

to carry us back to the origin of our system, or to a period when

its state was, in any great essential, different from what it is

at present. So far as the causes now in action go, and so far as

our calculations will enable us to estimate their effects, we are

equally unable to perceive in the general phenomena of the planetary

system either the evidence of a beginning, or the prospect of an end.

Geometers, as already stated, have demonstrated that, in the midst of

all the fluctuations which can possibly take place in the elements of

the orbits of the planets, by reason of their mutual attraction, the

general balance of the parts of the system will always be preserved,

and every departure from a mean state periodically compensated. But

neither the researches of the physical astronomer, nor those of

the geologist, give us any ground for regarding our system, or the

globe we inhabit, as of eternal duration. On the contrary, there are

circumstances in the physical constitution of our own planet which at

least obscurely point to an origin and a formation, however remote,

since it has been found that the figure of the earth is not globular

but elliptical, and that its attraction is such as requires us to admit

the interior to be more dense than the exterior, and the density to

increase with some degree of regularity from the surface towards the

centre, and _that_, in layers arranged elliptically round the centre,

circumstances which could scarcely happen without some such successive

deposition of materials as would enable pressure to be propagated with

a certain degree of freedom from one part of the mass to another, even

if we should hesitate to admit a state of primitive fluidity.

(314.) But from such indications nothing distinct can be concluded;

and if we would speculate to any purpose on a former state of our

globe and on the succession of events which from time to time may have

changed the condition and form of its surface, we must confine our

views within limits far more restricted, and to subjects much more

within the reach of our capacity, than either the creation of the world

or its assumption of its present figure. These, indeed, were favourite

speculations with a race of geologists now extinct; but the science

itself has undergone a total change of character, even within the last

half century, and is brought, at length, effectually within the list

of the inductive sciences. Geologists now no longer bewilder their

imaginations with wild theories of the formation of the globe from

chaos, or its passage through a series of hypothetical transformations,

but rather aim at a careful and accurate examination of the records

of its former state, which they find indelibly impressed on the great

features of its actual surface, and to the evidences of former life

and habitation which organised remains imbedded and preserved in its

strata indisputably afford.

(315.) Records of this kind are neither few nor vague; and though the

obsoleteness of their language when we endeavour to interpret it too

minutely, may, and no doubt often does, lead to misapprehension, still

its general meaning is, on the whole, unequivocal and satisfactory.

Such records teach us, in terms too plain to be misunderstood, that

the whole or nearly the whole of our present lands and continents

were formerly at the bottom of the sea, where they received deposits

of materials from the wearing and degradation of other lands not now

existing, and furnished receptacles for the remains of marine animals

and plants inhabiting the ocean above them, as well as for similar

spoils of the land washed down into its bosom.

(316.) These remains are occasionally brought to light; and their

examination has afforded indubitable evidence of the former existence

of a state of animated nature widely different from what now obtains

on the globe, and of a period anterior to that in which it has been

the habitation of man, or rather, indeed, of a series of periods, of

unknown duration, in which both land and sea teemed with forms of

animal and vegetable life, which have successively disappeared and

given place to others, and these again to new races approximating

gradually more and more nearly to those which now inhabit them, and at

length comprehending species which have their counterparts existing.

(317.) These wrecks of a former state of nature, thus wonderfully

preserved (like ancient medals and inscriptions in the ruins of an

empire), afford a sort of rude chronology, by whose aid the successive

depositions of the strata in which they are found may be marked out

in epochs more or less definitely terminated, and each characterized

by some peculiarity which enables us to recognise the deposits of any

period, in whatever part of the world they may be found. And, so far

as has been hitherto investigated, the _order_ of succession in which

these deposits have been formed appears to have been the same in every

part of the globe.

(318.) Many of the strata which thus bear evident marks of having been

deposited at the bottom of the sea, and of course in a horizontal

state, are now found in a position highly inclined to the horizon, and

even occasionally vertical. And they often bear no less evident marks

of violence, in their bending and fracture, the dislocation of parts

which were once contiguous, and the existence of vast collections of

broken fragments which afford every proof of great violence having been

used in accomplishing some at least of the changes which have taken

place.

(319.) Besides the rocks which carry this internal evidence of

submarine deposition, are many which exhibit no such proofs, but on the

contrary hold out every appearance of owing their origin to volcanoes

or to some other mode of igneous action; and in every part of the

world, and among strata of all ages, there occur evidences of such

action so abundant, and on such a scale, as to point out the volcano

and the earthquake as agents which may have been instrumental in the

production of those changes of level, and those violent dislocations

which we perceive to have taken place.

(320.) At all events, in accounting for those changes, geologists

have no longer recourse, as formerly, to causes purely hypothetical,

such as a shifting of the earth’s axis of rotation, bringing the sea

to overflow the land, by a change in the place of the longer and

shorter diameters of the spheroidal figure, nor to tides produced by

the attraction of comets suddenly approaching very near the earth,

nor to any other fanciful and arbitrarily assumed hypotheses; but

rather endeavour to confine themselves to a careful consideration

of causes evidently in action at present, with a view to ascertain

how far they, in the first instance, are capable of accounting for

the facts observed, and thus legitimately bringing into view, as

residual phenomena, those effects which cannot be so accounted for.

When this shall have been in some measure accomplished, we shall be

able to pronounce with greater security than at present respecting the

necessity of admitting a long succession of tremendous and ravaging

catastrophes and cataclysms,--epochs of terrific confusion and violence

which many geologists (perhaps with justice) regard as indispensable to

the explanation of the existing features of the world. We shall learn

to distinguish between the effects which require for their production

the sudden application of convulsive and fracturing efforts, and those,

probably not less extensive, changes which may have been produced by

forces equally or more powerful, but acting with less irregularity,

and so distributed over time as to produce none of those _interregnums_

of chaotic anarchy which we are apt to think (perhaps erroneously)

great disfigurements of an order so beautiful and harmonious as that of

nature.

(321.) But to estimate justly the effects of causes now in action in

geology is no easy task. There is no _à priori_ or deductive process by

which we can estimate the amount of the annual erosion, for instance,

of a continent by the action of meteoric agents, rain, wind, frost,

&c., nor the quantity of destruction produced on its coasts by the

direct violence of the sea, nor the quantity of lava thrown up _per

annum_ by volcanoes over the whole surface of the earth, nor any

similar effect. And to consult experience on all such points is a

slow and painful process if rightly gone into, and a very fallible

one if only partially executed. Much, then, at present must be left

to opinion, and to that sort of clear-judging tact which sometimes

anticipates experience; but this ought not to stand in the way of our

making every possible effort to obtain accurate information on such

points, by which alone geology can be rendered, if not an experimental

science, at least a science of that kind of active observation which

forms the nearest approach to it, where actual experiment is impossible.

(322.) Let us take, for example, the question, “What is the actual

direction in which changes of relative level are taking place between

the existing continents and seas?” If we consult partial experience,

that is, _all_ the information that we possess respecting ancient

sea-marks, soundings, &c., we shall only find ourselves bewildered in

a mass of conflicting, because imperfect, evidence. It is obvious that

the only way to decide the point is to ascertain, by very precise and

careful observations at proper stations on coasts, selected at points

where there exist natural marks not liable to change in the course of

at least a century, the true elevation of such marks above the _mean_

level of the sea, and to multiply these stations sufficiently over the

whole globe to be capable of affording real available knowledge. Now,

this is not a very easy operation (considering the accuracy required);

for the _mean_ level of the sea can be determined by no single

observation, any more than the mean height of the barometer at a given

station, being affected both by periodical and accidental fluctuations

due to tides, winds, waves, and currents. Yet if an instrument adapted

for the purpose were constructed, and rendered easily attainable, and

rules for its use carefully drawn up, there is little doubt we should

soon (by the industry of observers scattered over the world) be in

possession of a most valuable mass of information, which could not fail

to afford a point of departure for the next generation, and furnish

ground for the only kind of argument which ever can be conclusive on

such subjects.

(323.) Geology, in the magnitude and sublimity of the objects of

which it treats, undoubtedly ranks, in the scale of the sciences,

next to astronomy; like astronomy, too, its progress depends on the

continual accumulation of observations carried on for ages. But, unlike

astronomy, the observations on which it depends, when the whole extent

of the subject to be explored is taken into consideration, can hardly

yet be said to be more than commenced. Yet, to make up for this, there

is another important difference, that while in the latter science it is

impossible to recall the past or anticipate the future, and observation

is in consequence limited to a single fact in a single moment; in

the former, the records of the past are always present;--they may be

examined and re-examined as often as we please, and require nothing

but diligence and judgment to put us in possession of their whole

contents. Only a very small part of the surface of our globe has,

however, been accurately examined in detail, and of that small portion

we are only able to scratch the mere exterior, for so we must consider

those excavations which we are apt to regard as searching the bowels

of the earth; since the deepest mines which have been sunk penetrate

to a depth hardly surpassing the ten thousandth part of the distance

between its surface and its centre. Of course inductions founded on

such limited examination can only be regarded as provisional, except

in those remarkable cases where the same great formations in the same

order have been recognised in very distant quarters, and without

exception. This, however, cannot long be the case. The spirit with

which the subject has been prosecuted for many years in our own country

has been rewarded with so rich a harvest of surprising and unexpected

discoveries, and has carried the investigation of our island into such

detail, as to have excited a corresponding spirit among our continental

neighbours; while the same zeal which animates our countrymen on their

native shore accompanies them in their sojourns abroad, and has

already begun to supply a fund of information respecting the geology of

our Indian possessions, as well as of every other point where English

intellect and research can penetrate.

(324.) Nothing can be more desirable than that every possible facility

and encouragement should be afforded for such researches, and indeed

to the pursuits of the enlightened resident or traveller in every

department of science, by the representatives of our national authority

wherever our power extends. By these only can our knowledge of the

actual state of the surface of the globe, and that of the animals

and vegetables of the ancient continents and seas, be extended and

perfected, while more complete information than we at present possess

of the habits of those actually existing, and the influence of changes

of climate, food, and circumstances, on them, may be expected to render

material assistance to our speculations respecting those which have

become extinct.

CHAP. IV.

OF THE EXAMINATION OF THE MATERIAL CONSTITUENTS OF THE WORLD.

_Mineralogy._

(325.) The consideration of the history and structure of our globe, and

the examination of the fossil contents of its strata, lead us naturally

to consider the materials of which it consists. The history of these

materials, their properties as objects of philosophical enquiry, and

their application to the useful arts and the embellishments of life,

with the characters by which they can be certainly distinguished one

from another, form the object of mineralogy, taken in its most extended

sense.

(326.) There is no branch of science which presents so many points

of contact with other departments of physical research, and serves

as a connecting link between so many distant points of philosophical

speculation, as this. To the geologist, the chemist, the optician, the

crystallographer, the physician, it offers especially the very elements

of their knowledge, and a field for many of their most curious and

important enquiries. Nor, with the exception of chemistry, is there any

which has undergone more revolutions, or been exhibited in a greater

variety of forms. To the ancients it could scarcely be said to be at

all known, and up to a comparatively recent period, nothing could

be more imperfect than its descriptions, or more inartificial and

unnatural than its classification. The more important minerals in the

arts, indeed, those used for economical purposes and those from which

metals were extracted, had a certain degree of attention paid to them,

for the sake of their utility and commercial value, and the precious

stones for that of ornament. But until their crystalline forms were

attentively observed and shown to be determinate characters on which

dependence could be placed, no mineralogist could give any correct

account of the real distinction between one mineral and another.

(327.) It was only, however, when chemical analysis had acquired

a certain degree of precision and universal applicability that

the importance of mineralogy as a science began to be recognized,

and the connection between the external characters of a stone and

its ingredient constituents brought into distinct notice. Among

these characters, however, none were found to possess that eminent

distinctness which the crystalline form offers; a character, in the

highest degree geometrical, and affording, as might be naturally

supposed, the strongest evidence of its necessary connection with the

intimate constitution of the substance. The full importance of this

character was, however, not felt until its connection with the texture

or cleavage of a mineral was pointed out, and even then it required

numerous and striking instances of the critical discernment of Haüy

and other eminent mineralogists in predicting from the measurements

of the angles of crystals which had been confounded together that

differences would be found to exist in their chemical composition, all

which proved fully justified in their result before the essential value

of this character was acknowledged. This was no doubt in great measure

owing to the high importance set by the German mineralogists on those

external characters of touch, sight, weight, colour, and other sensible

qualities, which are little susceptible, with the exception of weight,

of exact determination, and which are subject to material variations

in different specimens of the same mineral. By degrees, however, the

necessity of ascribing great weight to a character so definite was

admitted, especially when it was considered that the same step which

pointed out the intimate connection of external form with internal

structure furnished the mineralogist with the means of reducing all

the forms of which a mineral is susceptible under one general type, or

primitive form, and afforded grounds for an elegant theoretical account

of the assumption of definite figures _ab initio_.

(328.) A simple and elegant invention of Dr. Wollaston, the reflecting

goniometer, gave a fresh impulse to that view of mineralogy which makes

the crystalline form the essential or leading character, by putting

it in the power of every one, by the examination of even the smallest

portion of a broken crystal, to ascertain and verify that essential

character on which the identity of a mineral in the system of Haüy

was made to depend. The application of so ready and exact a method

speedily led to important results, and to a still nicer discrimination

of mineral species than could before be attained; and the confirmation

given to these results by chemical analysis stamped them with a

scientific and decided character which they have retained ever since.

(329.) Meanwhile the progress made in chemical analysis had led to

the important conclusion that every chemical compound susceptible of

assuming the solid state assumed with it a determinate crystalline

form; and the progress of optical science had shown that the

fundamental crystalline form, in the case at least of transparent

bodies, drew with it a series of optical properties no less curious

than important in relation to the affections of light in its passage

through such substances. Thus, in every point of view, additional

importance became added to this character; and the study of the

crystalline forms of bodies in general assumed the form of a separate

and independent branch of science, of which the geometrical forms of

the mineral world constituted only a particular case. Mineralogy,

however, as a branch of natural history, remains still distinct either

from optics or crystallography. The mineralogist is content, and thinks

he has performed his task, if not as a natural historian at least as

a classifier and arranger, if he only gives such a characteristic

description of a mineral as shall effectually distinguish it from every

other, and shall enable any one who may encounter such a body in any

part of the world to impose on it its name, assign it a place in his

system, and turn to his books for a further description of all that

the chemist, the optician, the lapidary, or the artist, may require to

know. Still this is no easy matter: the laborious researches of the

most eminent mineralogists can hardly yet be said to have effectually

accomplished it; and its difficulty may be appreciated by the small

number of simple minerals, or minerals of perfectly definite and

well-marked characters, which have been hitherto made out. Nor can

this indeed be wondered at, when we consider that by far the greater

portion of the rocks and stones which compose the external crust of the

globe consists of nothing more than the accumulated _detritus_ of older

rocks, in which the fragments and powder of an infinite variety of

substances are mingled together, in all sorts of varying proportions,

and in such a way as to defy separation. Many of these rocks, however,

so compounded, occur with sufficient frequency and uniformity of

character to have acquired names and to have been usefully applied;

indeed, in the latter respect, minerals of this description far surpass

all the others. As objects of natural history, therefore, they are well

worthy of attention, however difficult it may be to assign them a place

in any artificial arrangement.

(330.) This paucity of simple minerals, however, is probably rather

apparent than real, and in proportion as the researches of the chemist

and crystallographer shall be extended throughout nature, they will

no doubt become much more numerous. Indeed, in the great laboratories

of nature it can hardly be doubted that almost every kind of chemical

process is going forwards, by which compounds of every description are

continually forming. Accordingly, it is remarked, that the lavas and

ejected scoriæ of volcanoes are receptacles in which mineral products

previously unknown are constantly discovered, and that the primitive

formations, as they are called in geology, which bear no marks of

having been produced by the destruction of others, are also remarkable

for the beauty and distinctness of character of their minerals.

(331.) The great difficulty which has been experienced in attempts to

classify mineral substances by their chemical constituents has arisen

from the observed presence, in some specimens of minerals bearing

that general resemblance in other respects as well as agreement in

form which would seem to entitle them to be considered as alike, of

ingredients foreign to the usual composition of the species, and that

occasionally in so large a proportion as to render it unjustifiable

to refer their occurrence to accidental impurities. These cases, as

well as some anomalies observed in the classification of minerals by

their crystalline forms, which seemed to show that the same substance

might occasionally appear under two distinct forms, as well as some

remarkable coincidences between the forms of substances quite distinct

from each other in a chemical point of view, have within a recent

period given rise to a branch of the science of crystallography of a

very curious and important nature. The _isomorphism_ of certain groups

of chemical elements has already afforded us an example illustrative

of the manner in which inductions sometimes receive unexpected

verifications (see 180.). The laws and relations thus brought to light

are among the most curious and interesting parts of modern science,

and seem likely in their further developement to afford ample scope

for the exercise of chemical and mineralogical research. They have

already afforded innumerable fine examples of that important step in

science by which anomalies disappear, and occasional incongruities

become reconciled under more general expressions of physical laws,

and thus unite in affording support to those very views which they

promised, when first observed, to overset. Nothing, indeed, can be

more striking than to see the very ingredient which every previous

chemist and mineralogist would agree to disregard and reject as a

mere casual impurity brought forward and appealed to in support of a

theory expressly directed to the object of rescuing science from the

imputation of disregarding, under any circumstances, the plain results

of direct experiment.

_Chemistry._

(332.) The laws which concern the intimate constitution of bodies,

not as respects their _structure_ or the manner in which their parts

are put together, but as regards their _materials_ or the ingredients

of which those parts are composed, form the objects of chemistry. A

solid body may be regarded as a fabric, more or less regularly and

artificially constructed, in which the materials and the workmanship

may be separately considered, and in which, though the latter be

ruined and confounded by violence, the former remain unchanged in

their nature, though differently arranged. In liquid or aërial bodies,

too, though there prevails a less degree of difference in point of

structure, and a greater facility of dispersion and dissipation, than

in solids, yet an equal diversity of _materials_ subsists, giving to

them properties differing extremely from each other.

(333.) The inherent activity of matter is proved not only by the

production of motion by the mutual attractions and repulsions of

distant or contiguous masses, but by the changes and apparent

transformations which different substances undergo in their sensible

qualities by mere mixture. If water be added to water, or salt to salt,

the effect is an increase of quantity, but no change of quality. In

this case, the mutual action of the particles is entirely mechanical.

Again, if a blue powder and a yellow one, each perfectly dry, be mixed

and well shaken together, a green powder will be produced; but this

is a mere effect arising in the eye from the intimate mixture of the

yellow and blue light separately and independently reflected from

the minute particles of each; and the proof is had by examining the

mixture with a microscope, when the yellow and blue grains will be seen

separate and each quite unaltered. If the same experiment be tried with

coloured liquids, which are susceptible of mixing without chemical

action, a compound colour is likewise produced, but no examination

with magnifiers is in that case sufficient to detect the ingredients;

the reason obviously being, the excessive minuteness of the parts, and

their perfect intermixture, produced by agitating two liquids together.

From the mixture of two powders, extreme patience would enable any one,

by picking out with a magnifier grain after grain, to separate the

ingredients. But when liquids are mixed, no mechanical separation is

any longer practicable; the particles are so minute as to elude all

search. Yet this does not hinder us from regarding such a compound as

still a mere mixture, and its properties are accordingly intermediate

between those of the liquids mixed. But this is far from being the

case with all liquids. When a solution of potash, for example, and

another of tartaric acid, each perfectly liquid, are mixed together in

proper proportions, a great quantity of a solid saline substance falls

to the bottom of the containing vessel, which is quite different from

either potash or tartaric acid, and the liquid from which it subsided

offers no indications by its taste or other sensible qualities of the

ingredients mixed, but of something totally different from either.

It is evident that this is a phenomenon widely different from that

of mere mixture; there has taken place a great and radical change in

the intimate nature of the ingredients, by which a new substance is

produced which had no existence before. And it has been produced by the

_union_ of the ingredients presented to each other; for when examined

it is found that nothing has been _lost_, the weight of the whole

mixture being the sum of the weights mixed. Yet the potash and tartaric

acid have disappeared entirely, and the weight of the new product is

found to be exactly equal to that of the tartaric acid and potash

employed, taken together, abating a small portion held in solution in

the liquid, which may be obtained however by evaporation. They have

therefore combined, and adhere to one another with a cohesive force

sufficient to form a solid out of a liquid; a force which has thus been

called into action by merely presenting them to each other in a state

of solution.

(334.) It is the business of chemistry to investigate these and similar

changes, or the reverse of such changes, where a single substance is

resolved into two or more others, having different properties from it,

and from each other, and to enquire into all the circumstances which

can influence them; and either determine, modify, or suspend their

accomplishment, whether such influence be exercised by heat or cold, by

time and rest, or by agitation or pressure, or by any of those agents

of which we have acquired a knowledge, such as electricity, light,

magnetism, &c.

(335.) The wonderful and sudden transformations with which chemistry is

conversant, the violent activity often assumed by substances usually

considered the most inert and sluggish, and, above all, the insight

it gives into the nature of innumerable operations which we see daily

carried on around us, have contributed to render it the most popular,

as it is one of the most extensively useful, of the sciences; and

we shall, accordingly, find none which have sprung forward, during

the last century, with such extraordinary vigour, and have had such

extensive influence in promoting corresponding progress in others. One

of the chief causes of its popularity is, perhaps, to be sought for in

this, that it is, of all the sciences, perhaps, the most completely an

experimental one; and even its theories are, for the most part, of that

generally intelligible and readily applicable kind, which demand no

intense concentration of thought, and lead to no profound mathematical

researches. The simple process of inductive generalization, grounded on

the examination of numerous facts, all of them presenting considerable

intrinsic interest, has sufficed, in most instances, to lead, by a

clear and direct road, to its highest laws yet known. But, on the other

hand, these laws, when stated, are not yet fully sufficient to lead us,

except in very limited cases, to a deductive knowledge of particulars

never before examined, at least, not without great caution, and

constant appeal to experiment as a check on our reasoning; so that we

are justified in regarding the _axioms_ of chemistry, the true handles

of deductive reasoning, as still unknown, and, perhaps, likely long

to remain so. This is no fault of its cultivators, who have comprised

in their list the highest and most varied talents and industry, but

of the inherent complexity of the subject, and the infinite multitude

of causes which are concerned in the production of every, even the

simplest, chemical phenomenon.

(336.) The history of chemistry (on which, however, we are not about

to enlarge,) is one of great interest to those who delight to trace

the steps by which mankind advance to the discovery of truth through

a series of mistakes and failures. It may be divided, 1st, into

the period of the alchemists, a lamentable epoch in the annals of

intellectual wandering; 2dly, that of the phlogistic doctrines of

Beccher and Stahl, in which, as if to prove the perversity of the human

mind, of two possible roads the wrong was chosen; and a theory obtained

universal credence on the strength of an induction, valid as such, but

wrongly interpreted, which is negatived, _in every instance_, by an

appeal to the balance. This, too, happened, not by reason of unlucky

coincidences, or individual oversights, but of necessity, and from an

inherent defect of the theory itself, which thus impeded the progress

of the science, as far as a science of experiment can be impeded by

a false theory, by perplexing its cultivators with the appearance of

contradictions in their experiments where none really subsisted, by

destroying all their confidence in the numerical exactness of their

own results, and by involving the subject in a mist of visionary and

hypothetical causes in place of the true acting principles. Thus, in

the combustion of any substance which is incapable of flying away in

fumes, an increase of weight takes place,--the ashes are heavier than

the fuel. Whenever this was observed, however, it was passed carelessly

over as arising from the escape of phlogiston, or the principle of

inflammability, which was considered as being either the element of

fire itself, or in some way combined with it, and thus essentially

_light_. It is now known that the increase of weight is owing to

the absorption of, and combination with, a quantity of a peculiar

ingredient called _oxygen_, from the air, a principle essentially

_heavy_. So far as weight is concerned, it makes no difference whether

a body having weight enters, or one having levity escapes; but there

is this plain difference in a philosophical point of view, that oxygen

is a real producible substance, and phlogiston is no such thing: the

former is a _vera causa_, the latter an hypothetical being, introduced

to account for what the other accounts for much better.

(337.) The third age of chemistry--that which may be called

emphatically modern chemistry--commenced (in 1786) when Lavoisier, by

a series of memorable experiments, extinguished for ever this error,

and placed chemistry in the rank of one of the exact sciences,--a

science of number, weight, and measure. From that epoch to the

present day it has constantly advanced with an accelerated progress,

and at this moment may be regarded as more progressive than ever.

The principal features in this progress may be comprised under the

following general heads:--

1. The discovery of the proximate, if not the ultimate, elements

of all bodies, and the enlargement of the list of known

elements to its present extent of between fifty and sixty

substances.

2. The developement of the doctrine of latent heat by Black,

with its train of important consequences, including the

scientific theory of the steam-engine.

3. The establishment of Wenzel’s law of definite proportions on

his own experiments, and those of Richter, a discovery

subsequently merged in the more general wording and better

development of Dalton’s atomic theory.

4. The precise determination of the atomic weights of the

different chemical elements, mainly due to the astonishing

industry of Berzelius, and his unrivalled command of chemical

resources, as well as to the researches of the other chemists

of the Swedish and German school.

5. The assimilation of gases and vapours, by which we are led

to regard the former, universally, as particular cases of

the latter, a generalization resulting chiefly from the

experiments of Faraday on the condensation of the gases,

and those of Gay-Lussac and Dalton, on the laws of their

expansion by heat compared with that of vapours.

6. The establishment of the laws of the combination of gases and

vapours by definite volumes, by Gay-Lussac.

7. The discovery of the chemical effects of electricity, and

the decomposing agency of the Voltaic pile, by Nicholson

and Carlisle; the investigation of the laws of such

decompositions, by Berzelius and Hisinger: the decomposition

of the alkalies by Davy, and the consequent introduction into

chemistry of new and powerful agents in their metallic bases.

8. The application of chemical analysis to all the objects of

organized and unorganized nature, and the discovery of

the ultimate constituents of all, and the proximate ones

of organic matter, and the recognisance of the important

distinctions which appear to divide these great classes of

bodies from each other.

9. The applications of chemistry to innumerable processes in the

arts, and among other useful purposes to the discovery of the

essential medical principles in vegetables, and to important

medicaments in the mineral kingdom.

10. The establishment of the intimate connection between

chemical composition and crystalline form, by Haüy

and Vauquelin, with the successive rectifications the

statement of that connection has undergone in the hands of

Mitscherlich, Rose, and others, with the progress of chemical

and crystallographical knowledge.

(338.) To pursue these several heads into detail would lead us into

a treatise on chemistry; but a few remarks on one or two of them, as

they bear upon the general principles of all scientific enquiry, will

not be irrelevant. And first, then, with reference to the discovery

of new elements, it will be observed, that philosophical chemistry no

more aims at determining the one essential element out of which all

matter is framed--the one ultimate principle of the universe--than

astronomy at discovering the origin of the planetary movements in

the application of a determinate projectile force in a determinate

direction, or geology at ascending to the creation of the earth. There

may be such an element. Some singular relations which have been pointed

out in the atomic weights of bodies seem to suggest to minds fond of

speculation that there is; but philosophical chemistry is content to

wait for some striking fact, which may either occur unexpectedly or

be led to by the slow progress of enlarged views, to disclose to us

its existence. Still, the multiplication of so-considered elementary

bodies has been considered by some as an inconvenience. We confess

we do not coincide with this view. Whatever they be, the obstinacy

with which they resist decomposition shows that they are ingredients

of a very high and primary importance in the economy of nature; and

such as, in any state of science, it would be indispensably necessary

to be perfectly familiar with. Like particular theorems in geometry,

which, though not rising to the highest point of generality, have yet

their several scopes and ranges of extensive application, they must be

well and perfectly understood in all their bearings. Should we ever

arrive at an analysis of these bodies, the chemical properties of the

new elements which will then come into view will be known only by our

knowledge of these, or of other compounds of the same class, which they

may be capable of forming. Not but that such an analysis would be a

most important and indeed triumphant achievement, and change the face

of chemistry; but it would undo nothing that has been done, and render

useless no point of knowledge which we have yet arrived at.

(339.) The atomic theory, or the law of definite proportions, which

is the same thing presented in a form divested of all hypothesis,

after the laws of mechanics, is, perhaps, the most important which

the study of nature has yet disclosed. The extreme simplicity which

characterizes it, and which is itself an indication, not unequivocal,

of its elevated rank in the scale of physical truths, had the effect

of causing it to be announced at once by Mr. Dalton, in its most

general terms, on the contemplation of a few instances[53], without

passing through subordinate stages of painful inductive ascent by the

intermedium of subordinate laws, such as, had the contrary course been

pursued by him, would have been naturally preparatory to it, and such

as would have led others to it by the prosecution of Wenzel’s and

Richter’s researches, had they been duly attended to. This is, in fact,

an example, and a most remarkable one, of the effect of that natural

propensity to generalize and simplify (noticed in 171.), which, if it

occasionally leads to over-hasty conclusions, limited or disproved by

further experience, is yet the legitimate parent of many of our most

valuable and soundest results. Instances like this, where great and,

indeed, immeasurable steps in our knowledge of nature are made at once,

and almost without intellectual effort, are well calculated to raise

our hopes of the future progress of science, and, by pointing out the

simplest and most obvious combinations as those which are actually

found to be agreeable to the harmony of creation, to hold out the

cheering prospect of difficulties diminishing as we advance, instead of

thickening around us in increasing complexity.

(340.) A consequence of this immediate presentation of the law of

definite proportions in its most general form is, that its subordinate

laws--those which limit its generality in particular cases, which

diminish the number of combinations abstractly possible, and restrain

the indiscriminate mixture of elements,--remain to be discovered. Some

such limitations have, in fact, been traced to a certain extent, but by

no means so far as the importance of the subject requires; and we have

here abundant occupation for chemists for some time.

(341.) The determination of the atomic weights of the chemical

elements, like that of other standard physical data, with the utmost

exactness, is in itself a branch of enquiry not only of the greatest

importance, but of extreme difficulty. Independent of the general

reasons for desiring accuracy in this respect, there is one peculiar

to the subject. It has been suggested (by Dr. Prout), and strongly

insisted on (by Dr. Thomson), that all the numbers representing these

weights, constituting a scale of great extent, in which the extremes

already known are in proportion to each other, as 1 to upwards of 200,

are simple even multiples of the least of them. If this be really the

case, it opens views of such importance as to justify any degree of

labour and pains in the verification of the law as a purely inductive

one. But in the actual state of chemical analysis, with all deference

to such high authority, we confess it appears to us to stand in great

need of further confirmation, since it seems doubtful whether such

accuracy has yet been attained as to enable us to answer positively

for a fraction not exceeding the three or four hundredth part of the

whole quantity to be determined: at least the results of the first

experimenters, obtained with the greatest care, differ often by a

greater amount; and this degree of exactness, at least, would be

required to verify the law satisfactorily in the higher parts of the

scale.

(342.) The mere agitation of such a question, however, points out

a class of phenomena in physical science of a remote and singular

kind, and of a very high and refined order, which could never become

known but in an advanced state of science, not only practical, but

theoretical,--we mean, such as consist in observed relations among the

_data_ of physics, which show them to be quantities not _arbitrarily_

assumed, but depending on laws and causes which they may be the means

of at length disclosing. A remarkable instance of such a relation is

the curious law which Bode observed to obtain in the progression of the

magnitudes of the several planetary orbits. This law was interrupted

between Mars and Jupiter, so as to induce him to consider a planet

as wanting in that interval;--a deficiency long afterwards strangely

supplied by the discovery of _four_ new planets in that very interval,

all of whose orbits conform in dimension to the law in question, within

such moderate limits of error as may be due to causes independent of

those on which the law itself ultimately rests.[54]

(343.) Neither is it irrelevant to our subject to remark, that the

progress which has been made in this department of chemistry, and

the considerable exactness actually attainable in chemical analysis,

have been owing, in great measure, to a circumstance which might at

first have been hardly considered likely to exercise much influence

on the progress of a science,--the discovery of platina. Without the

resources placed at the ready disposal of chemists by this invaluable

metal, it is difficult to conceive that the multitude of delicate

analytical experiments which have been required to construct the fabric

of existing knowledge could have ever been performed. This, among many

such lessons, will teach us that the most important uses of natural

objects are not those which offer themselves to us most obviously. The

chief use of the moon for man’s immediate purposes remained unknown to

him for five thousand years from his creation. And, since it cannot but

be that innumerable and most important uses remain to be discovered

among the materials and objects already known to us, as well as among

those which the progress of science must hereafter disclose, we may

hence conceive a well-grounded expectation, not only of constant

increase in the physical resources of mankind, and the consequent

improvement of their condition, but of continual accessions to our

power of penetrating into the arcana of nature, and becoming acquainted

with her highest laws.

CHAP. V.

OF THE IMPONDERABLE FORMS OF MATTER.

_Heat._

(344.) One of the chief agents in chemistry, on whose proper

application and management the success of a great number of its

enquiries depends, and many of whose most important laws are disclosed

to us by phenomena of a chemical nature, is HEAT. Although some of

its effects are continually before our eyes as matters of the most

common occurrence, insomuch that there is scarcely any process in the

useful arts and manufactures which does not call for its intervention,

and although, independent of this high utility, and the proportionate

importance of a knowledge of its nature and laws, it presents in itself

a subject of the most curious speculation; yet there is scarcely

any physical agent of which we have so imperfect a knowledge, whose

intimate nature is more hidden, or whose laws are of such delicate and

difficult investigation.

(345.) The word heat generally implies the sensation which we

experience on approaching a fire; but, in the sense it carries in

physics, it denotes the cause, whatever it be, of that sensation, and

of all the other phenomena which arise on the application of fire,

or of any other heating cause. We should be greatly deceived if we

referred only to sensation as an indication of the presence of this

cause. Many of those things which excite in our organs, and especially

of those of taste, a sensation of heat, owe this property to chemical

stimulants, and not at all to their being actually _hot_. This error

of judgment has produced a corresponding confusion of language, and

hence had actually at one period[55] crept into physical philosophy

a great many illogical and absurd conclusions. Again, there are a

number of chemical agents, which, from their corroding, blackening, and

dissolving, or drying up the parts of some descriptions of bodies, and

producing on them effects not generally unlike (though intrinsically

very different from) those produced by heat, are said, in loose and

vulgar language, to burn them; and this error has even become rooted

into a prejudice, by the fact that some of these agents are capable

of becoming actually and truly _hot_ during their action on moist

substances, by reason of their combination with the water the latter

contain. Thus, quicklime and oil of vitriol both exercise a powerful

corrosive action on animal and vegetable substances, and both become

violently hot by their combination with water. They are, therefore,

set down in vulgar parlance as substances of a hot nature; whereas,

in their relations to the physical cause of heat, they agree with the

generality of bodies similarly constituted.

(346.) The nature of heat has hitherto been chiefly studied under the

general heads of--

1st, Its sources, or the phenomena which it usually accompanies.

2d, Its communication from its sources to substances capable of

receiving it, and from these to others, with a view to

discover the laws which regulate its distribution through

space or through the bodies which occupy it.

3d, Its effects, on our senses, and on the bodies to which it is

communicated in its various degrees of intensity, by which,

means are afforded us of measuring these degrees.

4th, Its intimate relations to the atoms of matter, as exhibited

in its capability of acquiring a latent state under certain

circumstances, and of entering into something like chemical

combinations.

(347.) The most obvious sources of heat are, the sun, fire, animal

life, fermentations, violent chemical actions of all kinds, friction,

percussion, lightning, or the electric discharge, in whatever manner

produced, the sudden condensation of air, and others, so numerous,

and so varied, as to show the extensive and important part it has

to perform in the economy of nature. The discoveries of chemists,

however, have referred most of these to the general head of chemical

combination. Thus, fire, or the combustion of inflammable bodies, is

nothing more than a violent chemical action attending the combination

of their ingredients with the oxygen of the air. Animal heat is, in

like manner, referable to a process bearing no remote analogy to

a slow combustion, by which a portion of carbon, an inflammable

principle existing in the blood, is united with the oxygen of the air

in respiration; and thus carried off from the system: fermentation

is nothing more than a decomposition of chemical elements loosely

united, and their re-union in a more permanent state of combination.

The analogy between the sun and terrestrial fire is so natural as to

have been chosen by Newton to exemplify the irresistible force of an

inference derived from that principle. But the nature of the sun and

the mode in which its wonderful supply of light and heat is maintained

are involved in a mystery which every discovery that has been made

either in chemistry or optics, so far from elucidating, seems only to

render more profound. Friction as a source of heat is well known: we

rub our hands to warm them, and we grease the axles of carriage-wheels

to prevent their setting fire to the wood; an accident which, in

spite of this precaution, does sometimes happen. But the effect of

friction, as a means of producing heat with little or no consumption

of materials, was not fully understood till made the subject of direct

experiment by count Rumford, whose results appear to have established

the extraordinary fact, that an unlimited supply of heat may be derived

by friction from the same materials. Condensation, whether of air by

pressure, or of metals by percussion, is another powerful source of

heat. Thus, iron may be so dexterously hammered as to become red-hot,

and the rapid condensation of a confined portion of air will set tinder

on fire.

(348.) The most violent heats known are produced by the concentration

of the solar rays by burning glasses,--by the combustion of oxygen and

hydrogen gases mixed in the exact proportion in which they combine to

produce water,--and by the discharge of a continued and copious current

of electricity through a small conductor. As these three sources of

heat are independent of each other, and each capable of being brought

into action in a very confined space, there seems no reason why they

might not all three be applied at once at the same point, by which

means, probably, effects would be produced infinitely surpassing any

hitherto witnessed.

(349.) Heat is communicated either by _radiation_ between bodies at a

distance, or by _conduction_ between bodies in contact, or between the

contiguous parts of one and the same body. The laws of the radiation

of heat have been studied with great attention, and have been found

to present strong analogies with that of light in some points, and

singular differences in others. Thus, the heat which accompanies the

sun’s rays comports itself, in all respects, like light; being subject

to similar laws of reflection, refraction, and even of polarization, as

has been shown by Berard. Yet they are not identical with each other;

Sir William Herschel having shown, by decisive experiments, verified by

those of Sir H. Englefield, that there exist in a solar beam both rays

of heat which are not luminous, and rays of light which have no heating

power.

(350.) The heat, radiated by terrestrial fires, and by bodies

_obscurely_ hot, by whatever means they have acquired their heat

(even by exposure to the sun’s rays), differs very materially from

solar heat in their power of penetrating transparent substances. This

singular and important difference was first noticed by Mariotte, and

afterwards made the subject of many curious and interesting experiments

by Scheele, who found that terrestrial heat, or that radiated from

fires or heated bodies, is intercepted and detained by glass or

other transparent bodies, while solar heat is not; and that, being

so detained, it heats them: which the latter, as it passes freely

through them, is incapable of doing. The more recent researches of

Delaroche, however, have shown that this detention is complete only

when the temperature of the source of heat is low; but that, as that

temperature is higher, a portion of the heat radiated acquires a

power of penetrating glass; and that the quantity which does so bears

continually a larger and larger proportion to the whole, as the heat

of the radiant body is more intense. This discovery is very important,

as it establishes a community of nature between solar and terrestrial

heat; while at the same time it leads us to regard the actual

temperature of the sun as far exceeding that of any earthly flame.

(351.) A variety of theories have been framed to account for these

curious phenomena; but the subject stands rather in need of further

elucidation from experiment, and is one which merits, and will probably

amply repay, the labours of those who may hereafter devote their

attention to it. The theory of the radiation of heat, in general, which

seems to agree best with the known phenomena, is that of M. Prevost,

who considers all bodies as constantly radiating out heat in all

directions, and receiving it by a similar means of communication from

others, and thus tending, in any space filled, wholly or in part, with

bodies at various temperatures, to establish an equilibrium or equality

of heat in all parts. The application of this idea to the explanation

of the phenomenon of dew we have already seen (see 167.). The laws

of such radiation, under various circumstances, have been lately

investigated in a beautiful series of experiments on the cooling of

bodies by their own radiation in vacuo, by Messrs. Dulong and Petit,

which offer some of the best examples in science of the inductive

investigation of quantitative laws.

(352.) The communication of heat between bodies in contact, or between

the different parts of the same body, is performed by a process called

conduction. It is, in fact, only a particular case of radiation, as

has been explained above (217.); but a case _so_ particular as to

require a separate and independent investigation of its laws. The most

important consideration introduced into the enquiry by this peculiarity

is that of time. The communication of heat by conduction is performed,

for the most part, with extreme slowness, while that performed by

direct radiation is probably not less rapid than the propagation

of light itself. The analysis of the delicate and difficult points

which arise in the investigation of this subject in its reduction

to direct geometrical treatment has been executed with admirable

success by the late Baron Fourrier, whose recent lamented death has

deprived science of an ornament it could ill spare, thinned as its

ranks have been within the last few years. This acute philosopher and

profound mathematician has developed, in a series of elaborate memoirs

presented to the French Institute, the laws of the communication of

heat through the interior of solid masses, placed under the influence

of any external heating and cooling causes, and has in particular

applied his results to the conditions on which the maintenance of the

actual observed temperature on the earth’s surface depends; to the

possible influence of a supposed central heat on our climates; and to

the determination of the actual amount of the heat, derived to us from

the sun, or at least that portion of it on which the difference of the

seasons depends.

(353.) The principal effects of heat are the sensations of warmth or

cold consequent on its entry or egress into or out of our bodies; the

dilatation it causes in the dimensions of all substances in which it is

accumulated; the changes of state it produces in the melting of solids,

and the conversion of them and of liquids into vapour; and the chemical

changes it performs by actual decompositions effected in the intimate

molecules of various substances, especially those of which vegetables

and animals are composed; to which we may add, the production of

electric phenomena under certain circumstances in the contact of

metals, and the developement of electric polarity in crystallised

substances.

(354.) Cold has been considered by some as a positive quality, the

effect of a cause antagonist to that of heat; but this idea seems

now (with perhaps a single exception) to be universally abandoned.

The sensation of cold is as easily explicable by the passage of heat

outwards through the surface of the body as that of heat by its ingress

from without; and the experiments cited in proof of a radiation of

cold are all perfectly explained by Prevost’s theory of reciprocal

interchange. It is remarkable, however, how very limited our means

of producing intense cold are, compared with those we possess of

effecting the accumulation of heat in bodies. This is one of the

strongest arguments adducible in favour of the doctrines of those who

maintain the possibility of exhausting the heat of a body altogether,

and leaving it in a state absolutely devoid of it. But we ought to

consider, that the known methods of generating heat chiefly turn on the

production of chemical combinations: we may easily conceive, therefore,

that, to obtain equally powerful corresponding frigorific effects, we

ought to possess the means of effecting a disunion equally extensive

and rapid between such elements, actually combined, as have already

produced heat by their union. This, however, we can only accomplish by

engaging them in combinations still more energetic, that is to say,

in which we may reasonably expect more heat to be produced by the new

combination than would be destroyed or abstracted by the proposed

decomposition. Chemistry, however, (unaided by electric agency,)

affords no means of suddenly breaking the union of two elements, and

presenting _both_ in an uncombined state. A certain analogy to such

disunion, however, and its consequences, may be traced in the sudden

expansion of condensed gases from a liquid state into vapour, which is

the most powerful source of cold known.

(355.) The dilatation of bodies by heat forms the subject of that

branch of science called pyrometry. There is no body but is capable

of being penetrated by heat, though some with greater, others with

less rapidity; and being so penetrated, all bodies (with a very few

exceptions, and those depending on very peculiar circumstances,) are

dilated by it in bulk, though with a great diversity in the amount

of dilatation produced by the same degree of heat. Of the several

forms of natural bodies, gases and vapours are observed to be most

dilatable; liquids next, and solids least of all. The dilatation of

solids has been made a subject of repeated and careful measurement by

several experimenters; among whom, Smeaton, Lavoisier, and Laplace, are

the principal. The remarkable discovery of the unequal dilatation of

crystallised bodies by Mitscherlich has already been spoken of. (266.)

That of gases and vapours was examined about the same time by Dalton

and Gay-Lussac, who both arrived independently at the conclusion of an

equal dilatability subsisting in them all, which constitutes one of the

most remarkable points in their history.

(356.) The dilatation of air by heat affords, perhaps, the most

unexceptionable means known of measuring degrees of heat. The

thermometer, as originally constructed by Cornelius Drebell, was an

air thermometer. Those now in common use measure accessions of heat

not by the degree of dilatation of air but of mercury. It has been

shown, by the researches of Dulong and Petit, that its indications

coincide exactly with that of the air-thermometer in moderate

temperatures; though at very elevated ones they exhibit a sensible,

and even considerable, deviation. By this instrument, which owes its

present convenience and utility to the happy idea of Newton, who first

thought of fixing determinate points on its scale, we are enabled

to estimate, or at least identify, the degrees of heat; and thereby

to investigate with accuracy the laws of its communication and its

other properties. Were we sure that equal additions of heat produced

equal increments of dimension in any substance, the indications of a

thermometer would afford a true and secure _measure_ of the quantity

present; but this is so far from being the case, that we are nearly in

total ignorance on this important point; a circumstance which throws

the greatest difficulty in the way of all theoretical reasoning, and

even of experimental enquiry. The laws of the dilatation of liquids, in

consequence of this deficiency of necessary preliminary knowledge, are

still involved in great obscurity, notwithstanding the pains which have

been bestowed on them by the elaborate experiments and calculations of

Gilpin, Blagden, Deluc, Dalton, Gay-Lussac, and Biot.

(357.) The most striking and important of the effects of heat consist,

however, in the liquefaction of solid substances, and the conversion of

the liquids so produced into vapour. There is no solid substance known

which, by a sufficiently intense heat, may not be melted, and finally

dissipated in vapour; and this analogy is so extensive and cogent,

that we cannot but suppose that all those bodies which are liquid

under ordinary circumstances, owe their liquidity to heat, and would

freeze or become solid if their heat could be sufficiently reduced.

In many we see this to be the case in ordinary winters; for some,

severe frosts are requisite; others freeze only with the most intense

artificial colds; and some have hitherto resisted all our endeavours;

yet the number of these last is few, and they will probably cease to be

exceptions as our means of producing cold become enlarged.

(358.) A similar analogy leads us to conclude that all aëriform

fluids are merely liquids kept in the state of vapour by heat. Many

of them have been actually condensed into the liquid state by cold

accompanied with violent pressure; and as our means of applying these

causes of condensation have improved, more and more refractory ones

have successively yielded. Hence we are fairly entitled to extend our

conclusion to those which we have not yet been able to succeed with;

and thus we are led to regard it as a general fact, that the liquid and

aëriform or vaporous states are entirely dependent on _heat_; that were

it not for this cause, there would be nothing but solids in nature; and

that, on the other hand, nothing but a sufficient intensity of heat is

requisite to destroy the cohesion of every substance, and reduce all

bodies, first to liquids, and then into vapour.

(359.) But solids, themselves, by the abstraction of heat shrink in

dimension, and at the same time become harder, and more brittle;

yielding less to pressure, and permitting less separation between

their parts by tension. These facts, coupled with the greater

compressibility of liquids, and the still greater of gases, strongly

induce us to believe that it is heat, and heat alone, which holds

the particles of all bodies at that distance from each other which

is necessary to allow of compression; which in fact gives them

their elasticity, and acts as the antagonist force to their mutual

attraction, which would otherwise draw them into actual contact, and

retain them in a state of absolute immobility and impenetrability.

Thus we learn to regard heat as one of the great maintaining powers of

the universe, and to attach to all its laws and relations a degree of

importance which may justly entitle them to the most assiduous enquiry.

(360.) It was first ascertained by Dr. Black that when heat produces

the liquefaction of a solid, or the conversion of a liquid into vapour,

the liquid or the vapour resulting is no _hotter_ than the solid or

liquid from which it was produced, though a great deal of heat has been

expended in producing this effect, and has actually entered into the

substance.

(361.) Hence he drew the conclusion that it has become _latent_, and

continues to exist in the product, maintaining it in its new state,

without increasing its temperature. He further proved, that when the

vapour condenses, or the liquid freezes, this latent heat is again

given out from it. This great discovery, with its natural and hardly

less important concomitant, that of the difference of specific heats in

different bodies, or the different quantities of heat they require to

raise their temperature equally, are the chief reasons for regarding

heat as a material substance in a more decided manner than light, with

which in its radiant state it holds so close an analogy.

(362.) The subject of latent heat has been far less attentively

studied than its great practical importance would appear to demand,

when we consider that it is to this part of physical science that the

theory of the steam-engine is mainly referable, and that material

improvements may not unreasonably be expected in that wonderful

instrument, from a more extended knowledge than we possess of the

latent heats of different vapours. This is not the case, however,

with the subject of specific heat, which was followed up immediately

after its first promulgation with diligence by Irvine; and, after a

brief interval, by Lavoisier and Laplace, as well as by our countryman

Crawfurd, who determined the specific heats of many substances, both

solid and liquid. After a considerable period of inactivity, the

subject was again resumed by Delaroche and Berard, and subsequently

by Dulong and Petit: the result of whose investigations has been the

inductive establishment of one of those simple and elegant physical

laws which carry with them, if not their own evidence, at least their

own recommendation to our belief, as being in unison with every thing

we know of the harmony of nature. The law to which we allude is

this:--that the atoms of all the simple chemical elements have exactly

the same capacity for heat, or are all equally heated or cooled by

equal accessions or abstractions of heat. It is only among laws like

this that we can expect to find a clew capable of guiding us to a

knowledge of the true nature of heat, and its relations to ponderable

matter.

_Magnetism and Electricity._

(363.) These two subjects, which had long maintained a distinct

existence, and been studied as separate branches of science, are at

length effectually blended. This is, perhaps, the most satisfactory

result which the experimental sciences have ever yet attained. All

the phenomena of magnetic polarity, attraction, and repulsion, have

at length been resolved into one general fact, that two currents of

electricity, moving in the same direction repel, and in contrary

directions attract, each other. The phenomena of the communication

of magnetism and what is called its induced state, alone remain

unaccounted for; but the interesting theory which has been developed

by M. Ampere, under the name of Electro-dynamics, holds out a hope

that this difficulty will also in its turn give way, and the whole

subject be at length completely merged, as far as the consideration of

the acting causes goes, in the more general one of electricity. This,

however, does not prevent magnetism from maintaining its separate

importance as a department of physical enquiry, having its own peculiar

laws and relations of the highest practical interest, which are capable

of being studied quite apart from all consideration of its electrical

origin. And not only so, but to study them with advantage, we must

proceed as if that origin were totally unknown, and, at least up to

a certain point, and that a considerably advanced one, conduct our

enquiries into the subject on the same inductive principles as if this

branch of physics were absolutely independent of all others.

(364.) Iron, and its oxides and alloys, were for a long time the only

substances considered susceptible of magnetism. The loadstone was

even one of the examples produced by Bacon of that class of physical

instances to which he applies the term “Instantiæ monodicæ”--_singular

instances_. And the history of magnetism affords a beautiful comment

on his remark on instances of this sort. “Nor should our enquiries,”

he observes, “into their nature be broken off, till the properties and

qualities found in such things as may be esteemed wonders in nature

are reduced and comprehended under some certain law; so that all

irregularity or singularity may be found to depend upon some common

form, and the wonder only rest in the exact differences, degrees,

or extraordinary concurrence, and not in the species itself.” The

discovery of the magnetism of nickel, which though inferior to that

of iron, is still considerable; that of cobalt, yet feebler, and

that of titanium, which is only barely perceptible, have effectually

broken down the imaginary limit between iron and the other materials

of the world, and established the existence of that general law of

continuity which it is one chief business of philosophy to trace

throughout nature. The more recent discoveries of M. Arago (mentioned

in 160.) have completed this generalization, by showing that there

is no substance but which, under proper circumstances, is capable of

exhibiting unequivocal signs of the magnetic virtue. And to obliterate

all traces of that line of separation which was once so broad, we are

now enabled, by the great discovery of Oërsted, to communicate at and

during pleasure to a coiled wire of any metal indifferently all the

properties of a magnet;--its attraction, repulsion, and polarity; and

_that_ even in a more intense degree than was previously thought to

be possible in the best natural magnets. In short, in this case, and

in this case only, perhaps, in science, have we arrived at that point

which Bacon seems to have understood by the discovery of “forms.” “The

_form_ of any nature,” says he, “is such, that where it is, the given

nature must infallibly be. The form, therefore, is perpetually present

when that nature is present; ascertains it universally, and accompanies

it every where. Again, this form is such, that when removed, the given

nature infallibly vanishes. Lastly, a true form is such as can deduce

a given nature from some essential property, which resides in many

things.”