Miscellaneous Examples Of The Explanation Of Laws Of Nature.
§ 1. The most striking example which the history of science presents, of
the explanation of laws of causation and other uniformities of sequence
among special phenomena, by resolving them into laws of greater simplicity
and generality, is the great Newtonian generalization; respecting which
typical instance, so much having already been said, it is sufficient to
call attention to the great number and variety of the special observed
uniformities, which are in this case accounted for, either as particular
cases, or as consequences, of one very simple law of universal nature. The
simple fact of a tendency of every particle of matter toward every other
particle, varying inversely as the square of the distance, explains the
fall of bodies to the earth, the revolutions of the planets and
satellites, the motions (so far as known) of comets, and all the various
regularities which have been observed in these special phenomena; such as
the elliptical orbits, and the variations from exact ellipses; the
relation between the solar distances of the planets and the duration of
their revolutions; the precession of the equinoxes; the tides, and a vast
number of minor astronomical truths.
Mention has also been made in the preceding chapter of the explanation of
the phenomena of magnetism from laws of electricity; the special laws of
magnetic agency having been affiliated by deduction to observed laws of
electric action, in which they have ever since been considered to be
included as special cases. An example not so complete in itself, but even
more fertile in consequences, having been the starting-point of the really
scientific study of physiology, is the affiliation, commenced by Bichat,
and carried on by subsequent biologists, of the properties of the bodily
organs, to the elementary properties of the tissues into which they are
anatomically decomposed.
Another striking instance is afforded by Dalton’s generalization, commonly
known as the atomic theory. It had been known from the very commencement
of accurate chemical observation, that any two bodies combine chemically
with one another in only a certain number of proportions; but those
proportions were in each case expressed by a percentage—so many parts (by
weight) of each ingredient, in 100 of the compound (say 35 and a fraction
of one element, 64 and a fraction of the other); in which mode of
statement no relation was perceived between the proportion in which a
given element combines with one substance, and that in which it combines
with others. The great step made by Dalton consisted in perceiving that a
unit of weight might be established for each substance, such that by
supposing the substance to enter into all its combinations in the ratio
either of that unit, or of some low multiple of that unit, all the
different proportions, previously expressed by percentages, were found to
result. Thus 1 being assumed as the unit of hydrogen, if 8 were then taken
as that of oxygen, the combination of one unit of hydrogen with one unit
of oxygen would produce the exact proportion of weight between the two
substances which is known to exist in water; the combination of one unit
of hydrogen with two units of oxygen would produce the proportion which
exists in the other compound of the same two elements, called peroxide of
hydrogen; and the combinations of hydrogen and of oxygen with all other
substances, would correspond with the supposition that those elements
enter into combination by single units, or twos, or threes, of the numbers
assigned to them, 1 and 8, and the other substances by ones or twos or
threes of other determinate numbers proper to each. The result is that a
table of the equivalent numbers, or, as they are called, atomic weights,
of all the elementary substances, comprises in itself, and scientifically
explains, all the proportions in which any substance, elementary or
compound, is found capable of entering into chemical combination with any
other substance whatever.
§ 2. Some interesting cases of the explanation of old uniformities by
newly ascertained laws are afforded by the researches of Professor Graham.
That eminent chemist was the first who drew attention to the distinction
which may be made of all substances into two classes, termed by him
crystalloids and colloids; or rather, of all states of matter into the
crystalloid and the colloidal states, for many substances are capable of
existing in either. When in the colloidal state, their sensible properties
are very different from those of the same substance when crystallized, or
when in a state easily susceptible of crystallization. Colloid substances
pass with extreme difficulty and slowness into the crystalline state, and
are extremely inert in all the ordinary chemical relations. Substances in
the colloid state are almost always, when combined with water, more or
less viscous or gelatinous. The most prominent examples of the state are
certain animal and vegetable substances, particularly gelatine, albumen,
starch, the gums, caramel, tannin, and some others. Among substances not
of organic origin, the most notable instances are hydrated silicic acid,
and hydrated alumina, with other metallic peroxides of the aluminous
class.
Now it is found, that while colloidal substances are easily penetrated by
water, and by the solutions of crystalloid substances, they are very
little penetrable by one another: which enabled Professor Graham to
introduce a highly effective process (termed dialysis) for separating the
crystalloid substances contained in any liquid mixture, by passing them
through a thin septum of colloidal matter, which does not suffer any thing
colloidal to pass, or suffers it only in very minute quantity. This
property of colloids enabled Mr. Graham to account for a number of special
results of observation, not previously explained.
For instance, “while soluble crystalloids are always highly sapid, soluble
colloids are singularly insipid,” as might be expected; for, as the
sentient extremities of the nerves of the palate “are probably protected
by a colloidal membrane,” impermeable to other colloids, a colloid, when
tasted, probably never reaches those nerves. Again, “it has been observed
that vegetable gum is not digested in the stomach; the coats of that organ
dialyse the soluble food, absorbing crystalloids, and rejecting all
colloids.” One of the mysterious processes accompanying digestion, the
secretion of free muriatic acid by the coats of the stomach, obtains a
probable hypothetical explanation through the same law. Finally, much
light is thrown upon the observed phenomena of osmose (the passage of
fluids outward and inward through animal membranes) by the fact that the
membranes are colloidal. In consequence, the water and saline solutions
contained in the animal body pass easily and rapidly through the
membranes, while the substances directly applicable to nutrition, which
are mostly colloidal, are detained by them.(154)
The property which salt possesses of preserving animal substances from
putrefaction is resolved by Liebig into two more general laws, the strong
attraction of salt for water, and the necessity of the presence of water
as a condition of putrefaction. The intermediate phenomenon which is
interpolated between the remote cause and the effect, can here be not
merely inferred but seen; for it is a familiar fact, that flesh upon which
salt has been thrown is speedily found swimming in brine.
The second of the two factors (as they may be termed) into which the
preceding law has been resolved, the necessity of water to putrefaction,
itself affords an additional example of the Resolution of Laws. The law
itself is proved by the Method of Difference, since flesh completely dried
and kept in a dry atmosphere does not putrefy; as we see in the case of
dried provisions and human bodies in very dry climates. A deductive
explanation of this same law results from Liebig’s speculations. The
putrefaction of animal and other azotized bodies is a chemical process, by
which they are gradually dissipated in a gaseous form, chiefly in that of
carbonic acid and ammonia; now to convert the carbon of the animal
substance into carbonic acid requires oxygen, and to convert the azote
into ammonia requires hydrogen, which are the elements of water. The
extreme rapidity of the putrefaction of azotized substances, compared with
the gradual decay of non-azotized bodies (such as wood and the like) by
the action of oxygen alone, he explains from the general law that
substances are much more easily decomposed by the action of two different
affinities upon two of their elements than by the action of only one.
§ 3. Among the many important properties of the nervous system which have
either been first discovered or strikingly illustrated by Dr.
Brown-Séquard, I select the reflex influence of the nervous system on
nutrition and secretion. By reflex nervous action is meant, action which
one part of the nervous system exerts over another part, without any
intermediate action on the brain, and consequently without consciousness;
or which, if it does pass through the brain, at least produces its effects
independently of the will. There are many experiments which prove that
irritation of a nerve in one part of the body may in this manner excite
powerful action in another part; for example, food injected into the
stomach through a divided œsophagus, nevertheless produces secretion of
saliva; warm water injected into the bowels, and various other irritations
of the lower intestines, have been found to excite secretion of the
gastric juice, and so forth. The reality of the power being thus proved,
its agency explains a great variety of apparently anomalous phenomena; of
which I select the following from Dr. Brown-Séquard’s _Lectures on the
Nervous System_:
The production of tears by irritation of the eye, or of the mucous
membrane of the nose;
The secretions of the eye and nose increased by exposure of other parts of
the body to cold;
Inflammation of the eye, especially when of traumatic origin, very
frequently excites a similar affection in the other eye, which may be
cured by section of the intervening nerve;
Loss of sight sometimes produced by neuralgia, and has been known to be at
once cured by the extirpation (for instance) of a carious tooth;
Even cataract has been produced in a healthy eye by cataract in the other
eye, or by neuralgia, or by a wound of the frontal nerve;
The well-known phenomenon of a sudden stoppage of the heart’s action, and
consequent death, produced by irritation of some of the nervous
extremities; _e.g._, by drinking very cold water, or by a blow on the
abdomen, or other sudden excitation of the abdominal sympathetic nerve,
though this nerve may be irritated to any extent without stopping the
heart’s action, if a section be made of the communicating nerves;
The extraordinary effects produced on the internal organs by an extensive
burn on the surface of the body, consisting in violent inflammation of the
tissues of the abdomen, chest, or head, which, when death ensues from this
kind of injury, is one of the most frequent causes of it;
Paralysis and anæsthesia of one part of the body from neuralgia in another
part; and muscular atrophy from neuralgia, even when there is no
paralysis;
Tetanus produced by the lesion of a nerve. Dr. Brown-Séquard thinks it
highly probable that hydrophobia is a phenomenon of a similar nature;
Morbid changes in the nutrition of the brain and spinal cord, manifesting
themselves by epilepsy, chorea, hysteria, and other diseases, occasioned
by lesion of some of the nervous extremities in remote places, as by
worms, calculi, tumors, carious bones, and in some cases even by very
slight irritations of the skin.
§ 4. From the foregoing and similar instances, we may see the importance,
when a law of nature previously unknown has been brought to light, or when
new light has been thrown upon a known law by experiment, of examining all
cases which present the conditions necessary for bringing that law into
action; a process fertile in demonstrations of special laws previously
unsuspected, and explanations of others already empirically known.
For instance, Faraday discovered by experiment, that voltaic electricity
could be evolved from a natural magnet, provided a conducting body were
set in motion at right angles to the direction of the magnet; and this he
found to hold not only of small magnets, but of that great magnet, the
earth. The law being thus established experimentally, that electricity is
evolved, by a magnet, and a conductor moving at right angles to the
direction of its poles, we may now look out for fresh instances in which
these conditions meet. Wherever a conductor moves or revolves at right
angles to the direction of the earth’s magnetic poles, there we may expect
an evolution of electricity. In the northern regions, where the polar
direction is nearly perpendicular to the horizon, all horizontal motions
of conductors will produce electricity; horizontal wheels, for example,
made of metal; likewise all running streams will evolve a current of
electricity, which will circulate round them; and the air thus charged
with electricity may be one of the causes of the Aurora Borealis. In the
equatorial regions, on the contrary, upright wheels placed parallel to the
equator will originate a voltaic circuit, and water-falls will naturally
become electric.
For a second example, it has been proved, chiefly by the researches of
Professor Graham, that gases have a strong tendency to permeate animal
membranes, and diffuse themselves through the spaces which such membranes
inclose, notwithstanding the presence of other gases in those spaces.
Proceeding from this general law, and reviewing a variety of cases in
which gases lie contiguous to membranes, we are enabled to demonstrate or
to explain the following more special laws: 1st. The human or animal body,
when surrounded with any gas not already contained within the body,
absorbs it rapidly; such, for instance, as the gases of putrefying
matters: which helps to explain malaria. 2d. The carbonic acid gas of
effervescing drinks, evolved in the stomach, permeates its membranes, and
rapidly spreads through the system. 3d. Alcohol taken into the stomach
passes into vapor, and spreads through the system with great rapidity
(which, combined with the high combustibility of alcohol, or in other
words its ready combination with oxygen, may perhaps help to explain the
bodily warmth immediately consequent on drinking spirituous liquors). 4th.
In any state of the body in which peculiar gases are formed within it,
these will rapidly exhale through all parts of the body; and hence the
rapidity with which, in certain states of disease, the surrounding
atmosphere becomes tainted. 5th. The putrefaction of the interior parts of
a carcass will proceed as rapidly as that of the exterior, from the ready
passage outward of the gaseous products. 6th. The exchange of oxygen and
carbonic acid in the lungs is not prevented, but rather promoted, by the
intervention of the membrane of the lungs and the coats of the
blood-vessels between the blood and the air. It is necessary, however,
that there should be a substance in the blood with which the oxygen of the
air may immediately combine; otherwise, instead of passing into the blood,
it would permeate the whole organism: and it is necessary that the
carbonic acid, as it is formed in the capillaries, should also find a
substance in the blood with which it can combine; otherwise it would leave
the body at all points, instead of being discharged through the lungs.
§ 5. The following is a deduction which confirms, by explaining, the
empirical generalization, that soda powders weaken the human system. These
powders, consisting of a mixture of tartaric acid with bicarbonate of
soda, from which the carbonic acid is set free, must pass into the stomach
as tartrate of soda. Now, neutral tartrates, citrates, and acetates of the
alkalis are found, in their passage through the system, to be changed into
carbonates; and to convert a tartrate into a carbonate requires an
additional quantity of oxygen, the abstraction of which must lessen the
oxygen destined for assimilation with the blood, on the quantity of which
the vigorous action of the human system partly depends.
The instances of new theories agreeing with and explaining old
empiricisms, are innumerable. All the just remarks made by experienced
persons on human character and conduct, are so many special laws, which
the general laws of the human mind explain and resolve. The empirical
generalizations on which the operations of the arts have usually been
founded, are continually justified and confirmed on the one hand, or
corrected and improved on the other, by the discovery of the simpler
scientific laws on which the efficacy of those operations depends. The
effects of the rotation of crops, of the various manures, and other
processes of improved agriculture, have been for the first time resolved
in our own day into known laws of chemical and organic action, by Davy,
Liebig, and others. The processes of the medical art are even now mostly
empirical: their efficacy is concluded, in each instance, from a special
and most precarious experimental generalization: but as science advances
in discovering the simple laws of chemistry and physiology, progress is
made in ascertaining the intermediate links in the series of phenomena,
and the more general laws on which they depend; and thus, while the old
processes are either exploded, or their efficacy, in so far as real,
explained, better processes, founded on the knowledge of proximate causes,
are continually suggested and brought into use.(155) Many even of the
truths of geometry were generalizations from experience before they were
deduced from first principles. The quadrature of the cycloid is said to
have been first effected by measurement, or rather by weighing a cycloidal
card, and comparing its weight with that of a piece of similar card of
known dimensions.
§ 6. To the foregoing examples from physical science, let us add another
from mental. The following is one of the simple laws of mind: Ideas of a
pleasurable or painful character form associations more easily and
strongly than other ideas, that is, they become associated after fewer
repetitions, and the association is more durable. This is an experimental
law, grounded on the Method of Difference. By deduction from this law,
many of the more special laws which experience shows to exist among
particular mental phenomena may be demonstrated and explained: the ease
and rapidity, for instance, with which thoughts connected with our
passions or our more cherished interests are excited, and the firm hold
which the facts relating to them have on our memory; the vivid
recollection we retain of minute circumstances which accompanied any
object or event that deeply interested us, and of the times and places in
which we have been very happy or very miserable; the horror with which we
view the accidental instrument of any occurrence which shocked us, or the
locality where it took place and the pleasure we derive from any memorial
of past enjoyment; all these effects being proportional to the sensibility
of the individual mind, and to the consequent intensity of the pain or
pleasure from which the association originated. It has been suggested by
the able writer of a biographical sketch of Dr. Priestley in a monthly
periodical,(156) that the same elementary law of our mental constitution,
suitably followed out, would explain a variety of mental phenomena
previously inexplicable, and in particular some of the fundamental
diversities of human character and genius. Associations being of two
sorts, either between synchronous, or between successive impressions; and
the influence of the law which renders associations stronger in proportion
to the pleasurable or painful character of the impressions, being felt
with peculiar force in the synchronous class of associations; it is
remarked by the writer referred to, that in minds of strong organic
sensibility synchronous associations will be likely to predominate,
producing a tendency to conceive things in pictures and in the concrete,
richly clothed in attributes and circumstances, a mental habit which is
commonly called Imagination, and is one of the peculiarities of the
painter and the poet; while persons of more moderate susceptibility to
pleasure and pain will have a tendency to associate facts chiefly in the
order of their succession, and such persons, if they possess mental
superiority, will addict themselves to history or science rather than to
creative art. This interesting speculation the author of the present work
has endeavored, on another occasion, to pursue further, and to examine how
far it will avail toward explaining the peculiarities of the poetical
temperament.(157) It is at least an example which may serve, instead of
many others, to show the extensive scope which exists for deductive
investigation in the important and hitherto so imperfect Science of Mind.
§ 7. The copiousness with which the discovery and explanation of special
laws of phenomena by deduction from simpler and more general ones has here
been exemplified, was prompted by a desire to characterize clearly, and
place in its due position of importance, the Deductive Method; which, in
the present state of knowledge, is destined henceforth irrevocably to
predominate in the course of scientific investigation. A revolution is
peaceably and progressively effecting itself in philosophy, the reverse of
that to which Bacon has attached his name. That great man changed the
method of the sciences from deductive to experimental, and it is now
rapidly reverting from experimental to deductive. But the deductions which
Bacon abolished were from premises hastily snatched up, or arbitrarily
assumed. The principles were neither established by legitimate canons of
experimental inquiry, nor the results tested by that indispensable element
of a rational Deductive Method, verification by specific experience.
Between the primitive method of Deduction and that which I have attempted
to characterize, there is all the difference which exists between the
Aristotelian physics and the Newtonian theory of the heavens.
It would, however, be a mistake to expect that those great
generalizations, from which the subordinate truths of the more backward
sciences will probably at some future period be deduced by reasoning (as
the truths of astronomy are deduced from the generalities of the Newtonian
theory), will be found in all, or even in most cases, among truths now
known and admitted. We may rest assured, that many of the most general
laws of nature are as yet entirely unthought of; and that many others,
destined hereafter to assume the same character, are known, if at all,
only as laws or properties of some limited class of phenomena; just as
electricity, now recognized as one of the most universal of natural
agencies, was once known only as a curious property which certain
substances acquired by friction, of first attracting and then repelling
light bodies. If the theories of heat, cohesion, crystallization, and
chemical action are destined, as there can be little doubt that they are,
to become deductive, the truths which will then be regarded as the
_principia_ of those sciences would probably, if now announced, appear
quite as novel(158) as the law of gravitation appeared to the
contemporaries of Newton; possibly even more so, since Newton’s law, after
all, was but an extension of the law of weight—that is, of a
generalization familiar from of old, and which already comprehended a not
inconsiderable body of natural phenomena. The general laws of a similarly
commanding character, which we still look forward to the discovery of, may
not always find so much of their foundations already laid.
These general truths will doubtless make their first appearance in the
character of hypotheses; not proved, nor even admitting of proof, in the
first instance, but assumed as premises for the purpose of deducing from
them the known laws of concrete phenomena. But this, though their initial,
can not be their final state. To entitle an hypothesis to be received as
one of the truths of nature, and not as a mere technical help to the human
faculties, it must be capable of being tested by the canons of legitimate
induction, and must actually have been submitted to that test. When this
shall have been done, and done successfully, premises will have been
obtained from which all the other propositions of the science will
thenceforth be presented as conclusions, and the science will, by means of
a new and unexpected Induction, be rendered Deductive.