Humphry Davy, the celebrated natural philosopher, was born Dec. 17,
1778, at Penzance, England. At the age of seventeen he became an
apothecary's apprentice, and at the age of nineteen assistant at
Dr. Beddoes's pneumatic institution at Bristol. During researches
at the pneumatic institution he discovered the physiological
effects of "laughing gas," and made so considerable a reputation as
a chemist that at the age of twenty-two he was appointed lecturer,
and a year later professor, at the Royal Institution. For ten
years, from 1803, he was engaged in agricultural researches, and in
1813 published his "Elements of Agricultural Chemistry." During the
same decade he conducted important investigations into the nature
of chemical combination, and succeeded in isolating the elements
potassium, sodium, strontium, magnesium, and chlorine. In 1812 he
was knighted, and married Mrs. Apreece, _née_ Jane Kerr. In 1815 he
investigated the nature of fire-damp and invented the Davy safety
lamp. In 1818 he received a baronetcy, and two years later was
elected President of the Royal Society. On May 29, 1829, he died at
Geneva. Davy's "Elements of Chemical Philosophy," of which a
summary is given here, was published in one volume in 1812, being
the substance of lectures delivered before the Board of
Agriculture.
_I.--Forms and Changes of Matter_
The forms and appearances of the beings and substances of the external
world are almost infinitely various, and they are in a state of
continued alteration. In general, matter is found in four forms, as (1)
solids, (2) fluids, (3) gases, (4) ethereal substances.
1. _Solids._ Solids retain whatever mechanical form is given to them;
their parts are separated with difficulty, and cannot readily be made to
unite after separation. They may be either elastic or non-elastic, and
differ in hardness, in colour, in opacity, in density, in weight, and,
if crystalline, in crystalline form.
2. _Fluids._ Fluids, when in small masses, assume the spherical form;
their parts possess freedom of motion; they differ in density and
tenacity, in colour, and in opacity. They are usually regarded as
incompressible; at least, a very great mechanical force is required to
compress them.
3. _Gases._ Gases exist free in the atmosphere, but may be confined.
Their parts are highly movable; they are compressible and expansible,
and their volumes are inversely as the weight compressing them. All
known gases are transparent, and present only two or three varieties of
colour; they differ materially in density.
4. _Ethereal Substances._ Ethereal substances are known to us only in
their states of motion when acting upon our organs of sense, or upon
other matter, and are not susceptible of being confined. It cannot be
doubted that there is such matter in motion in space. Ethereal matter
differs either in its nature, or in its affections by motion, for it
produces different effects; for instance, radiant heat, and different
kinds of light.
All these forms of matter are under the influence of active forces, such
as gravitation, cohesion, heat, chemical and electrical attraction, and
these we must now consider.
1. _Gravitation._ When a stone is thrown into the atmosphere, it rapidly
descends towards the earth. This is owing to gravitation. All the great
bodies in the universe are urged towards each other by a similar force.
Bodies mutually gravitate towards each other, but the smaller body
proportionately more than the larger one; hence the power of gravity is
said to vary directly as the mass. Gravitation also varies with
distance, and acts inversely as the square of the distance.
2. _Cohesion._ Cohesion is the force which preserves the forms of
solids, and gives globularity to fluids. It is usually said to act only
at the surface of bodies or by their immediate contact; but this does
not seem to be the case. It certainly acts with much greater energy at
small distances, but the spherical form of minute portions of fluid
matter can be produced only by the attractions of all the parts of which
they are composed, for each other; and most of these attractions must be
exerted at sensible distances, so that gravitation and cohesion may be
mere modifications of the same general power of attraction.
3. _Heat._ When a body which occasions the sensation of heat on our
organs is brought into contact with another body which has no such
effect, the hot body contracts and loses to a certain extent its power
of communicating heat; and the other body expands. Different solids and
fluids expand very differently when heated, and the expansive power of
liquids, in general, is greater than that of solids.
It is evident that the density of bodies must be diminished by
expansion; and in the case of fluids and gases, the parts of which are
mobile, many important phenomena depend upon this circumstance. For
instance, if heat be applied to fluids and gases, the heated parts
change their places and rise, and the currents in the ocean and
atmosphere are due principally to this movement. There are very few
exceptions to the law of the expansion of bodies at the time they become
capable of communicating the sensation of heat, and these exceptions
seem to depend upon some chemical change in the constitution of bodies,
or on their crystalline arrangements.
The power which bodies possess of communicating or receiving heat is
known as _temperature_, and the temparature of a body is said to be high
or low with respect to another in proportion as it occasions an
expansion or contraction of its parts.
When equal volumes of different bodies of different temperatures are
suffered to remain in contact till they acquire the same temperature, it
is found that this temperature is not a mean one, as it would be in the
case of equal volumes of the same body. Thus if a pint of quicksilver
at 100° be mixed with a pint of water at 50°, the resulting temperature
is not 75°, but 70°; the mercury has lost thirty degrees, whereas the
water has only gained twenty degrees. This difference is said to depend
on the different _capacities_ of bodies for heat.
Not only do different bodies vary in their capacity for heat, but they
likewise acquire heat with very different degrees of celerity. This last
difference depends on the different power of bodies for _conducting_
heat, and it will be found that as a rule the densest bodies, with the
least capacity for heat, are the best conductors.
Heat, or the power of repulsion, may be considered as the _antagonist_
power to the attraction of cohesion. Thus solids by a certain increase
of temperature become fluids, and fluids gases; and, _vice versâ_, by a
diminution of temperature, gases become fluids, and fluids solids.
Proofs of the conversion of solids, fluids, or gases into ethereal
substances are not distinct. Heated bodies become luminous and give off
radiant heat, which affects the bodies at a distance, and it may
therefore be held that particles are thrown off from heated bodies with
great velocity, which, by acting on our organs, produce the sensations
of heat or light, and that their motion, communicated to the particles
of other bodies, has the power of expanding them. It may, however, be
said that the radiant matters emitted by bodies in ignition are specific
substances, and that common matter is not susceptible of assuming this
form; or it may be contended that the phenomena of radiation do in fact,
depend upon motions communicated to subtile matter everywhere existing
in space.
The temperatures at which bodies change their states from fluids to
solids, though in general definite, are influenced by a few
circumstances such as motion and pressure.
When solids are converted into fluids, or fluids into gases, there is
always a loss of heat of temperature; and, _vice versâ_, when gases are
converted into fluids, or fluids into solids, there is an increase of
heat of temperature, and in this case it is said that _latent_ heat is
absorbed or given out.
The expansion due to heat has been accounted for by supposing a subtile
fluid, or _caloric_, capable of combining with bodies and of separating
their parts from each other, and the absorption and liberation of latent
heat can be explained on this principle. But many other facts are
incompatible with the theory. For instance, metal may be kept hot for
any length of time by friction, so that if _caloric_ be pressed out it
must exist in an inexhaustible quantity. Delicate experiments have shown
that bodies, when heated, do not increase in weight.
It seems possible to account for all the phenomena of heat, if it be
supposed that in solids the particles are in a constant state of
vibratory motion, the particles of the hottest bodies moving with the
greatest velocity and through the greatest space; that in fluids and
gases the particles have not only vibratory motion, but also a motion
round their own axes with different velocities, and that in ethereal
substances the particles move round their own axes and separate from
each other, penetrating in right lines through space. Temperature may be
conceived to depend upon the velocity of the vibrations, increase of
capacity on the motion being performed in greater space; and the
diminution of temperature during the conversion of solids into fluids or
gases may be explained on the idea of the loss of vibratory motion in
consequence of the revolution of particles round their axes at the
moment when the body becomes fluid or aeriform, or from the loss of
rapidity of vibration in consequence of the motion of particles through
greater space.
4. _Chemical Attraction._ Oil and water will not _combine_; they are
said to have no chemical _attraction_ or _affinity_ for each other. But
if oil and solution of potassa in water be mixed, the oil and the
solution blend and form a soap; and they are said to attract each other
chemically or to have a _chemical affinity_ for each other. It is a
general character of chemical combination that it changes the qualities
of the bodies. Thus, corrosive and pungent substances may become mild
and tasteless; solids may become fluids, and solids and fluids gases.
No body will act chemically upon another body at any sensible distance;
apparent contact is necessary for chemical action. A freedom of motion
in the parts of the bodies or a want of cohesion greatly assists action,
and it was formerly believed that bodies cannot act chemically upon each
other unless one of them be fluid or gaseous.
Different bodies unite with different degrees of force, and hence one
body is capable of separating others from certain of their combinations,
and in consequence mutual decompositions of different compounds take
place. This has been called _double affinity_, or _complex chemical
affinity_.
As in all well-known compounds the proportions of the elements are in
certain definite ratios to each other, it is evident that these ratios
may be expressed by numbers; and if one number be employed to denote the
smallest quantity in which a body combines, all other quantities of the
same body will be multiples of this number, and the smallest proportions
into which the undecomposed bodies enter into union being known, the
constitution of the compounds they form may be learnt, and the element
which unites chemically in the smallest quantity being expressed by
unity, all the other elements may be represented by the relations of
their quantities to unity.
5. _Electrical Attraction._ A piece of dry silk briskly rubbed against a
warm plate of polished flint glass acquires the property of adhering to
the glass, and both the silk and the glass, if apart from each other,
attract light substances. The bodies are said to be _electrically
excited_. Probably, all bodies which differ from each other become
electrically excited when rubbed and pressed together. The electrical
excitement seems of two kinds. A pith-ball touched by glass excited by
silk repels a pith-ball touched by silk excited by metals. Electrical
excitement of the same nature as that in glass excited by silk is known
as _vitreous_ or _positive_, and electrical excitement of the opposite
nature is known as _resinous_ or _negative_.
A rod of glass touched by an electrified body is electrified only round
the point of contact. A rod of metal, on the contrary, suspended on a
rod of glass and brought into contact with an electrical surface,
instantly becomes electrical throughout. The glass is said to be a
_non-conductor_, or _insulating substance_; the metal a _conductor_.
When a non-conductor or imperfect conductor, provided it be a thin plate
of matter placed upon a conductor, is brought in contact with an excited
electrical body, the surface opposite to that of contact gains the
opposite electricity from that of the excited body, and if the plate be
removed it is found to possess two surfaces in opposite states. If a
conductor be brought into the neighbourhood of an excited body--the air,
which is a non-conductor, being between them--that extremity of the
conductor which is opposite to the excited body gains the opposite
electricity; and the other extremity, if opposite to a body connected
with the ground, gains the same electricity, and the middle point is not
electrical at all. This is known as _induced_ electricity.
The common exhibition of electrical effects is in attractions and
repulsions; but electricity also produces chemical phenomena. If a piece
of zinc and copper in contact with each other at one point be placed in
contact at other points with the same portion of water, the zinc will
corrode, and attract oxygen from the water much more rapidly than if it
had not been in contact with the copper; and if sulphuric acid be added,
globules of inflammable air are given off from the copper, though it is
not dissolved or acted upon.
Chemical phenomena in connection with electrical effects can be shown
even better by combinations in which the electrical effects are
increased by alterations of different metals and fluids--the so-called
_voltaic batteries_. Such are the decomposing powers of such batteries
that not even insoluble compounds are capable of resisting their energy,
for even glass, sulphate of baryta, fluorspar, etc., are slowly acted
upon, and the alkaline, earthy, or acid matter carried to the poles in
the common order.
The most powerful voltaic combinations are formed by substances that act
chemically with most energy upon each other, and such substances as
undergo no chemical changes in the combination exhibit no electrical
powers. Hence it was supposed that the electrical powers of metals were
entirely due to chemical changes; but this is not the case, for contact
produces electricity even when no chemical change can be observed.
_II.--Radiant or Ethereal Matter_
When similar thermometers are placed in different parts of the solar
beam, it is found that different effects are produced in the differently
coloured rays. The greatest heat is exhibited in the red rays, the least
in the violet rays; and in a space beyond the red rays, where there is
no visible light, the increase of temperature is greatest of all.
From these facts it is evident that matter set in motion by the sun has
the power of producing heat without light, and that its rays are less
refrangible than the visible rays. The invisible rays that produce heat
are capable of reflection as well as refraction in the same manner as
the visible rays.
Rays capable of producing heat with and without light proceed not only
from the sun, but also from bodies at the surface of the globe under
peculiar agencies or changes. If, for instance, a thermometer be held
near an ignited body, it receives an impression connected with an
elevation of temperature; this is partly produced by the conducting
powers of the air, and partly by an impulse which is instantaneously
communicated, even to a considerable distance. This effect is called the
radiation of terrestrial heat.
The manner in which the temperatures of bodies are affected by rays
producing heat is different for different substances, and is very much
connected with their colours. The bodies that absorb most light, and
reflect least, are most heated when exposed either to solar or
terrestrial rays. Black bodies are, in general, more heated than red;
red more than green; green more than yellow; and yellow more than white.
Metals are less heated than earthy or stony bodies, or than animal or
vegetable matters. Polished surfaces are less heated than rough
surfaces.
The bodies that have their temperatures most easily raised by heat rays
are likewise those that are most easily cooled by their own radiation,
or that at the same temperature emit most heat-making rays. Metals
radiate less heat than glass, glass less than vegetable substances, and
charcoal has the highest radiating powers of any body as yet made the
subject of experiment.
Radiant matter has the power of producing chemical changes partly
through its heating power, and partly through some other specific and
peculiar influence. Thus chlorine and hydrogen detonate when a mixture
of them is exposed to the solar beams, even though the heat is
inadequate to produce detonation.
If moistened silver be exposed to the different rays of the solar
spectrum, it will be found that no effect is produced upon it by the
least refrangible rays which occasion heat without light; that a slight
discoloration only will be produced by the red rays; that the effect of
blackening will be greater towards the violet end of the spectrum; and
that in a space beyond the violet, where there is no sensible heat or
light, the chemical effect will be very distinct. There seem to be rays,
therefore, more refrangible than the rays producing light and heat.
The general facts of the refraction and effects of the solar beam offer
an analogy to the agencies of electricity.
In general, in Nature the effects of the solar rays are very compounded.
Healthy vegetation depends upon the presence of the solar beams or of
light, and while the heat gives fluidity and mobility to the vegetable
juices, chemical effects are likewise occasioned, oxygen is separated
from them, and inflammable compounds are formed. Plants deprived of
light become white and contain an excess of saccharine and aqueous
particles; and flowers owe the variety of their hues to the influence of
the solar beams. Even animals require the presence of the rays of the
sun, and their colours seem to depend upon the chemical influence of
these rays.
Two hypotheses have been invented to account for the principal
operations of radiant matter. In the first it is supposed that the
universe contains a highly rare elastic substance, which, when put into
a state of undulation, produces those effects on our organs of sight
which constitute the sensations of vision and other phenomena caused by
solar and terrestrial rays. In the second it is conceived that particles
are emitted from luminous or heat-making bodies with great velocity, and
that they produce their effects by communicating their motions to
substances, or by entering into them and changing their composition.
Newton has attempted to explain the different refrangibility of the rays
of light by supposing them composed of particles differing in size. The
same great man has put the query whether light and common matter are not
convertible into each other; and, adopting the idea that the phenomena
of sensible heat depend upon vibrations of the particles of bodies,
supposes that a certain intensity of vibrations may send off particles
into free space, and that particles in rapid motion in right lines, in
losing their own motion, may communicate a vibratory motion to the
particles of terrestrial bodies.