Molecular Forces—Permanency of the ultimate Particles of
Matter—Interstices—Mossotti’s Theory—Rankin’s Theory of Molecular
Vortices—Gases reduced to Liquids by Pressure—Gravitation of
Particles—Cohesion—Crystallization—Cleavage—Isomorphism—Minuteness of
the Particles—Height of Atmosphere—Chemical Affinity—Definite
Proportions and Relative Weights of Atoms—Faraday’s Discovery with
regard to Affinity—Capillary Attraction.
THE oscillations of the atmosphere, and its action upon the rays of
light coming from the heavenly bodies, connect the science of astronomy
with the equilibrium and movements of fluids and the laws of molecular
attraction. Hitherto that force has been under consideration which acts
upon masses of matter at sensible distances; but now the effects of such
forces are to be considered as act at inappreciable distances upon the
ultimate molecules of material bodies.
All substances consist of an assemblage of material particles, or
molecules, which are far too small to be visible by any means human
ingenuity has yet been able to devise, and which are much beyond the
limits of our perceptions. They neither can be created nor destroyed;
bodies may be burned, but their particles are not consumed—they are
merely liberated from one combination to enter into another, nor are
their peculiar properties ever changed; whatever combinations they may
enter into, they are ever and invariably the same.
Since every known substance may be reduced in bulk by pressure, it
follows that the particles of matter are not in actual contact, but are
separated by interstices; and it is evident that the smaller the
interstitial spaces the greater the density. These spaces appear to be
filled with air in some cases, as may be inferred from certain
semi-opaque minerals and other substances becoming transparent when
plunged into water. Sometimes they may possibly contain some unknown and
highly elastic fluid, such as Sir David Brewster has discovered in the
minute cavities of various minerals, which occasionally causes them to
explode under the hands of the lapidary; but as it is inconceivable that
the particles of matter should act upon one another without some means
of communication, it is presumed that the interstices of material
substances contain a portion of the ethereal medium with which the
regions of space are filled.
The various hypotheses that have been formed as to the nature and action
of the forces which unite the particles of matter, have been
successively given up as science advanced, and now nothing decisive has
been attained, although Professor Mossotti, of Pisa, by a very able
analysis, has endeavoured to prove the identity of the cohesive force
with gravitation. As the particles of material bodies are not in actual
contact, he supposes that each is surrounded by an atmosphere of the
ethereal medium, which he conceives to be electricity; moreover he
assumes that the atoms of the medium repel one another, that the
particles of matter also repel one another, but with less intensity, and
that there is a mutual attraction between the particles of matter and
the atoms of the medium, forces which are assumed to vary inversely as
the square of the distance.
Hence, when the material molecules of a body are inappreciably near to
one another, they mutually repel each other with a force which
diminishes rapidly as the infinitely small distance between the material
molecules augments, and at last vanishes. When the molecules are still
farther apart, the force becomes attractive. At that particular point
where the change takes place the forces of repulsion and attraction
balance each other, so that the molecules of a body are neither disposed
to approach nor recede, but remain in equilibrio. If we try to press
them nearer, the repulsive force resists the attempt; and if we
endeavour to break the body so as to tear the particles asunder, the
attractive force predominates and keeps them together. This is what
constitutes the cohesive force, or force of aggregation, by which the
molecules of all substances are united. The limits of the distance at
which the negative action becomes positive vary according to the
temperature and nature of the molecules, and determine whether the body
which they form be solid, liquid, or aëriform.
Beyond this neutral point the attractive force increases as the distance
between the molecules augments till it attains a maximum; when the
particles are more apart, it diminishes; and, as soon as they are
separated by finite or sensible distances, it varies directly as their
mass and inversely as the square of the distance, which is precisely the
law of universal gravitation.
Thus, on the hypothesis that the mutual repulsion between the electric
atoms is a little more powerful than the mutual repulsion between the
particles of matter, the ether and the matter attract each other with
unequal intensities, which leaves an excess of attractive force
constituting gravitation. As the gravitating force is in operation
wherever there is matter, the ethereal electric medium must encompass
all the bodies in the universe; and, as it is utterly incomprehensible
that the celestial bodies should exert a reciprocal attraction through a
void, the Professor concludes that the ethereal electrical medium fills
all space.
It is true that this connexion between the molecular forces and
gravitation depends upon hypothesis; but in the greater number of
physical investigations some hypothesis is requisite in the first
instance to aid the imperfection of our senses; and when the phenomena
of nature accord with the assumption, we are justified in believing it
to be a general law.
Mr. Rankin’s theory of molecular vortices, or the molecular structure of
matter, is independent of electricity. According to his hypothesis, each
atom of matter consists of an inappreciably small nucleus, encompassed
by an elastic ethereal atmosphere which is retained in its position by
attractive forces directed towards the molecule, whilst the molecules
attract each other in the direction of straight lines joining their
centres. The nuclei may either be solid, or a high condensation of the
atmospheres which surround each with decreasing density. When the
attraction between the molecules is such that the elasticity of the
atmospheres is insensible, the body is a perfect solid, the rigidity of
which bears a certain definite proportion to the elasticity of the
volume. When the atmospheres are less condensed and the attraction of
the molecules merely produces a cohesive force sufficient to balance the
atomic elasticity of the atmosphere, the body is a perfect liquid; and
when the attraction of the molecules is very small compared with the
elasticity of their ethereal atmospheres, the body is a perfect gas.
These atmospheres are supposed to be portions of the ethereal medium
which penetrates into the interstices of every substance, and their
elasticity to be due to the heat generated by the centrifugal force or
oscillations among their atoms, for motion is the cause of heat, the
force producing the motions varying simply as the density of the ether.
In aëriform fluids, although the particles are more remote from each
other than in liquids and solids, yet the pressure may be so great as to
reduce an aëriform fluid to a liquid, and a liquid to a solid. Dr.
Faraday has reduced some of the gases to a liquid state by very great
compression; but although atmospheric air is capable of a diminution of
volume to which we do not know a limit, it has hitherto always retained
its gaseous qualities, and resumes its primitive volume the instant the
pressure is removed. Substances are said to be more or less elastic,
according to the facility with which they regain their bulk or volume
when the pressure is removed; thus liquids resist compression on account
of their elasticity, and in solids the resistance is much greater but
variable, and the effort required to break a substance is a measure of
the cohesive force exerted by its particles. In stone, iron, steel, and
all brittle and hard substances, the cohesion of the particles is
powerful but of small extent; in elastic bodies, on the contrary, its
action is weak, but more extensive. An infinite variety of conditions
may be observed in the fusion of metals and other substances passing
from hardness to toughness, viscidity, and through all the other stages
to perfect fluidity and even to vapour. Since all bodies expand by heat,
the cohesive force is weakened by increase of temperature. The cohesion
of matter or the strength of substances forms an important branch of
study in engineering.
Every particle of matter, whether it forms a constituent part of a
solid, liquid, or aëriform fluid, is subject to the law of gravitation.
The weight of the atmosphere, of gases and vapour, shows that they
consist of gravitating particles. In liquids the cohesive force is not
sufficiently powerful to resist the action of gravitation. Therefore,
although their component particles still maintain their connexion, the
liquid is scattered by their weight, unless when it is confined in a
vessel or has already descended to the lowest point possible, and
assumed a level surface from the mobility of its particles and the
influence of the gravitating forces, as in the ocean, or a lake. Solids
would also fall to pieces by the weight of their particles, if the force
of cohesion were not powerful enough to resist the efforts of
gravitation.
The phenomena arising from the force of cohesion are innumerable. The
spherical form of rain-drops; the difficulty of detaching a plate of
glass from the surface of water; the force with which two plane surfaces
adhere when pressed together; the drops that cling to the window-glass
in a shower of rain—are all effects of cohesion entirely independent of
atmospheric pressure, and are included in the same analytical formula
(N. 162) which expresses all the circumstances accurately, although the
laws according to which the forces of cohesion and repulsion vary are
unknown. It is more than probable that the spherical form of the sun and
planets is due to the force of cohesion, as they have every appearance
of having been at one period in a state of fusion.
A very remarkable instance has occasionally been observed in plate-glass
manufactories. After the large plates of glass of which mirrors are to
be made have received their last polish, they are carefully wiped and
laid on their edges with their surfaces resting on one another. In the
course of time the cohesion has sometimes been so powerful, that they
could not be separated without breaking. Instances have occurred where
two or three have been so perfectly united, that they have been cut and
their edges polished as if they had been fused together; and so great
was the force required to make the surfaces slide that one tore off a
portion of the surface of the other.
In liquids and gases the forms of the particles have no influence, they
are so far apart; but the structure of solids varies according to the
sides which the particles present to one another during their
aggregation. Nothing is known of their form further than the
dissimilarity of their different sides in certain cases, which appears
from their reciprocal attractions during crystallisation being more or
less powerful according to the sides they present to one another.
Crystallisation is an effect of molecular attraction regulated by
certain laws, according to which atoms of the same kind of matter unite
in regular forms—a fact easily proved by dissolving a piece of alum in
pure water. The mutual attraction of the particles is destroyed by the
water; but, if it be evaporated, they unite, and form in uniting
eight-sided figures called octahedrons (N. 163). These however are not
all the same. Some have their angles cut off, others their edges, and
some both, while the remainder take the regular form. It is quite clear
that the same circumstances which cause the aggregation of a few
particles would, if continued, cause the addition of more; and the
process would go on as long as any particles remain free round the
primitive nucleus, which would increase in size, but would remain
unchanged in form, the figure of the particles being such as to maintain
the regularity and smoothness of the surfaces of the solid and their
mutual inclinations. A broken crystal will by degrees resume its regular
figure when put back again into the solution of alum, which shows that
the internal and external particles are similar, and have a similar
attraction for the particles held in solution. The original conditions
of aggregation which make the molecules of the same substance unite in
different forms must be very numerous, since of carbonate of lime alone
there are many hundred varieties; and certain it is, from the motion of
polarised light through rock crystal, that a very different arrangement
of particles is requisite to produce an extremely small change in
external form. A variety of substances in crystallising combine
chemically with a certain portion of water which in a dry state forms an
essential part of their crystals, and, according to the experiments of
MM. Haidinger and Mitscherlich, seems in some cases to give the peculiar
determination to their constituent molecules. These gentlemen have
observed that the same substance crystallising at different temperatures
unites with different quantities of water and assumes a corresponding
variety of forms. Seleniate of zinc, for example, unites with three
different portions of water, and assumes three different forms,
according as its temperature in the act of crystallising is hot,
lukewarm, or cold. Sulphate of soda also, which crystallises at 90° of
Fahrenheit without water of crystallisation, combines with water at the
ordinary temperature, and takes a different form. Heat appears to have a
great influence on the phenomena of crystallisation, not only when the
particles of matter are free, but even when firmly united, for it
dissolves their union, and gives them another determination. Professor
Mitscherlich found that prismatic crystals of sulphate of nickel
(N. 164), exposed to a summer’s sun in a close vessel, had their
internal structure so completely altered without any exterior change,
that when broken open they were composed internally of octahedrons with
square bases. The original aggregation of the internal particles had
been dissolved, and a disposition given to arrange themselves in a
crystalline form. Crystals of sulphate of magnesia and of sulphate of
zinc, gradually heated in alcohol till it boils, lose their transparency
by degrees, and when opened are found to consist of innumerable minute
crystals totally different in form from the whole crystals; and
prismatic crystals of zinc (N. 165) are changed in a few seconds into
octahedrons by the heat of the sun: other instances might be given of
the influence of even moderate degrees of temperature on molecular
attraction in the interior of substances. It must be observed that these
experiments give entirely new views with regard to the constitution of
solid bodies. We are led from the mobility of fluids to expect great
changes in the relative positions of their molecules, which must be in
perpetual motion even in the stillest water or calmest air; but we were
not prepared to find motion to such an extent in the interior of solids.
That their particles are brought nearer by cold and pressure, or removed
farther from one another by heat, might be expected; but it could not
have been anticipated that their relative positions could be so entirely
changed as to alter their mode of aggregation. It follows, from the low
temperature at which these changes are effected, that there is probably
no portion of inorganic matter that is not in a state of relative
motion.
Professor Mitscherlich’s discoveries with regard to the forms of
crystallised substances, as connected with their chemical character,
have thrown additional light on the constitution of material bodies.
There is a certain set of crystalline forms which are not susceptible of
variation, as the die or cube (N. 166), which may be small or large, but
is invariably a solid bounded by six square surfaces or planes. Such
also is the tetrahedron (N. 167) or four-sided solid contained by four
equal-sided triangles. Several other solids belong to this class, which
is called the Tessular system of crystallisation. There are other
crystals which, though bounded by the same number of sides, and having
the same form, are yet susceptible of variation; for instance, the
eight-sided figure with a square base, called an octahedron (N. 168),
which is sometimes flat and low, and sometimes acute and high. It was
formerly believed that identity of form in all crystals not belonging to
the Tessular system indicated identity of chemical composition.
Professor Mitscherlich, however, has shown that substances differing to
a certain degree in chemical composition have the property of assuming
the same crystalline form. For example, the neutral phosphate of soda
and the arseniate of soda crystallise in the very same form, contain the
same quantities of acid, alkali, and water of crystallisation; yet they
differ so far, that one contains arsenic and the other an equivalent
quantity of phosphorus. Substances having such properties are said to be
isomorphous, that is, equal in form. Of these there are many groups,
each group having the same form, and similarity though not identity of
chemical composition. For instance, one of the isomorphous groups is
that consisting of certain chemical substances called the protoxides of
iron, copper, zinc, nickel, and manganese, all of which are identical in
form and contain the same quantity of oxygen, but differ in the
respective metals they contain, which are, however, nearly in the same
proportion in each. All these circumstances tend to prove that
substances having the same crystalline form must consist of ultimate
atoms having the same figure and arranged in the very same order; so
that the form of crystals is dependent on their atomic constitution.
All crystallised bodies have joints called cleavages, at which they
split more easily than in other directions; on this property the whole
art of cutting diamonds depends. Each substance splits in a manner and
in forms peculiar to itself. For example, all the hundreds of forms of
carbonate of lime split into six-sided figures, called rhombohedrons
(N. 169), whose alternate angles measure 105·55° and 75·05°, however far
the division may be carried; therefore the ultimate particle of
carbonate of lime is presumed to have that form. However this may be, it
is certain that all the various crystals of that mineral may be formed
by building up six-sided solids of the form described, in the same
manner as children build houses with miniature bricks. It may be
imagined that a wide difference may exist between the particles of an
unformed mass and a crystal of the same substance—between the common
shapeless limestone and the pure and limpid crystal of Iceland spar; yet
chemical analysis detects none; their ultimate atoms are identical, and
crystallisation shows that the difference arises only from the mode of
aggregation. Besides, all substances either crystallise naturally, or
may be made to do so by art. Liquids crystallise in freezing, vapours by
sublimation (N. 170); and hard bodies, when fused, crystallise in
cooling. Hence it may be inferred that all substances are composed of
atoms, on whose magnitude, density, and form, their nature and qualities
depend; and, as these qualities are unchangeable, the ultimate particles
of matter must be incapable of wear—the same now as when created.
The size of the ultimate particles of matter must be small in the
extreme. Organised beings, possessing life and all its functions, have
been discovered so small, that a million of them would occupy less space
than a grain of sand. The malleability of gold, the perfume of musk, the
odour of flowers, and many other instances might be given of the
excessive minuteness of the atoms of matter. Supposing the density of
the air at the surface of the earth to be represented by unity, Sir John
Herschel has shown that, under any hypothesis as to its atoms, it would
require a fraction having at least 1370 figures in its denominator to
express its tenuity in the interplanetary space; yet the definite
proportions of chemical compounds afford a proof that divisibility of
matter has a limit. The cohesive force, which has been the subject of
the preceding considerations, only unites particles of the same kind of
matter; whereas affinity, which is the cause of chemical compounds, is
the mutual attraction between particles of different kinds of matter,
generally producing a compound which has no sensible property in common
with its component parts except that of their combined gravity, as, for
example, water, which is a compound of oxygen and hydrogen gases. It is
merely a result of the electrical state of the particles, chemical
affinity and electricity being only forms of the same power. In most
cases it produces electricity, as in the oxidation of metals and
combustion, and in every case without exception heat is evolved by
bodies while combining chemically; and as heat is an expansive force,
chemical action is changed into mechanical expansion, but it is not
known in this case why heat is produced, nor the manner in which the
particles act.
It is a permanent and universal law in vast numbers of unorganised
bodies that their composition is definite and invariable, the same
compound always consisting of the same elements united together in the
same proportions. Two substances may indeed be mixed; but they will not
combine to form a third substance different from both, unless their
component particles unite in definite proportions; that is to say, one
part by weight of one of the substances will unite with one part by
weight of the other, or with two parts, or three, or four, &c., so as to
form a new substance; but in any other proportions they will only be
mechanically mixed. For example, one part by weight of hydrogen gas will
combine with eight parts by weight of oxygen gas, and form water; or it
will unite with sixteen parts by weight of oxygen, and form a substance
called deutoxide of hydrogen; but, added to any other weight of oxygen,
it will produce one or both of these compounds mingled with the portion
of oxygen or hydrogen in excess. The law of definite proportion
established by Dr. Dalton, on the principle that every compound body
consists of a combination of the atoms of its constituent parts, is of
universal application, and is in fact one of the most important
discoveries in physical science, furnishing information previously
unhoped for with regard to the most secret and minute operations of
nature, in disclosing the relative weights of the ultimate atoms of
matter. Thus an atom of oxygen uniting with an atom of hydrogen forms
the compound water; but, as every drop of water however small consists
of eight parts by weight of oxygen and one part by weight of hydrogen,
it follows that an atom of oxygen is eight times heavier than an atom of
hydrogen. In the same manner sulphuretted hydrogen gas consists of
sixteen parts by weight of sulphur and one of hydrogen; therefore an
atom of sulphur is sixteen times heavier than an atom of hydrogen. Also
carbonic oxide is constituted of six parts by weight of carbon and eight
of oxygen; and, as an atom of oxygen has eight times the weight of an
atom of hydrogen, it follows that an atom of carbon is six times heavier
than one of hydrogen. Since the same definite proportion holds in the
composition of a vast number of substances that have been examined, it
has been concluded that there are great differences in the weights of
the ultimate particles of matter. Although Dalton’s law is fully
established, yet instances have occurred from which it appears that the
atomic theory deduced from it is not always maintained. M. Gay Lussac
discovered that gases unite together by their bulk or volumes, in such
simple and definite proportions as one to one, one to two, one to three,
&c. For example, one volume or measure of oxygen unites with two volumes
or measures of hydrogen in the formation of water.
Dr. Faraday has proved, by experiments on bodies both in solution and
fusion, that chemical affinity is merely a result of the electrical
state of the particles of matter. Now it must be observed that the
composition of bodies, as well as their decomposition, may be
accomplished by means of electricity; and Dr. Faraday has found that
this chemical composition and decomposition, by a given current of
electricity, is always accomplished according to the laws of definite
proportions; and that the quantity of electricity requisite for the
decomposition of a substance is exactly the quantity necessary for its
composition. Thus the quantity of electricity which can decompose a
grain weight of water is exactly equal to the quantity of electricity
which unites the elements of that grain of water together, and is
equivalent to the quantity of atmospheric electricity which is active in
a very powerful flash of lightning. This law is universal, and of that
high and general order which characterises all great discoveries.
Chemical force is extremely powerful. A pound of the best coal gives
when burnt sufficient heat to raise the temperature of 8086 pounds of
water one Centigrade degree, whence Professor Helmholtz of Bonn has
computed that the magnitude of the chemical force of attraction between
the particles of a pound of coal and the quantity of oxygen that
corresponds to it, is capable of lifting a weight of 100 pounds to the
height of 20 miles.
Dr. Faraday has given a singular instance of cohesive force inducing
chemical combination, by the following experiment, which seems to be
nearly allied to the discovery made by M. Dœbereiner, in 1823, of the
spontaneous combustion of spongy platinum (N. 171) exposed to a stream
of hydrogen gas mixed with common air. A plate of platinum with
extremely clean surfaces, when plunged into oxygen and hydrogen gas
mixed in the proportions which are found in the constitution of water,
causes the gases to combine and water to be formed, the platinum to
become red-hot, and at last an explosion to take place; the only
conditions necessary for this curious experiment being excessive purity
in the gases and in the surface of the plate. A sufficiently pure
metallic surface can only be obtained by immersing the platinum in very
strong hot sulphuric acid and then washing it in distilled water, or by
making it the positive pole of a galvanic pile in dilute sulphuric acid.
It appears that the force of cohesion, as well as the force of affinity,
exerted by particles of matter, extends to all the particles within a
very minute distance. Hence the platinum, while drawing the particles of
the two gases towards its surface by its great cohesive attraction,
brings them so near to one another that they come within the sphere of
their mutual affinity, and a chemical combination takes place. Dr.
Faraday attributes the effect in part also to a diminution in the
elasticity of the gaseous particles on their sides adjacent to the
platinum, and to their perfect mixture or association, as well as to the
positive action of the metal in condensing them against its surface by
its attractive force. The particles when chemically united run off the
surface of the metal in the form of water by their gravitation, or pass
away as aqueous vapour and make way for others.
The oscillations of the atmosphere, and the changes in its temperature,
are measured by variations in the heights of the barometer and
thermometer. But the actual length of the liquid columns depends not
only upon the force of gravitation, but upon the cohesive force or
reciprocal attraction between the molecules of the liquid and those of
the tube containing it. This peculiar action of the cohesive force is
called capillary attraction or capillarity. If a glass tube of extremely
fine bore, such as a small thermometer tube, be plunged into a cup of
water or spirit of wine, the liquid will immediately rise in the tube
above the level of that in the cup; and the surface of the little column
thus suspended will be a hollow hemisphere, whose diameter is the
interior diameter of the tube. If the same tube be plunged into a cupful
of mercury, the liquid will also rise in the tube, but it will never
attain the level of that in the cup, and its surface will be a
hemisphere whose diameter is also the diameter of the tube (N. 172). The
elevation or depression of the same liquid in different tubes of the
same matter is in the inverse ratio of their internal diameters
(N. 173), and altogether independent of their thickness; whence it
follows that the molecular action is insensible at sensible distances,
and that it is only the thinnest possible film of the interior surface
of the tubes that exerts a sensible action on the liquid. So much indeed
is this the case, that, when tubes of the same bore are completely
wetted with water throughout their whole extent, mercury will rise to
the same height in all of them, whatever be their thickness or density,
because the minute coating of moisture is sufficient to remove the
internal column of mercury beyond the sphere of attraction of the tube,
and to supply the place of a tube by its own capillary attraction. The
forces which produce the capillary phenomena are the reciprocal
attraction of the tube and the liquid, and of the liquid particles on
one another; and, in order that the capillary column may be in
equilibrio, the weight of that part of it which rises above or sinks
below the level of the liquid in the cup must balance these forces.
The estimation of the action of the liquid is a difficult part of this
problem. La Place, Dr. Young, and other mathematicians, have considered
the liquid within the tube to be of uniform density; but M. Poisson, in
one of those masterly productions in which he elucidates the most
abstruse subjects, has proved that the phenomena of capillary attraction
depend upon a rapid decrease in the density of the liquid column
throughout an extremely small space at its surface. Every indefinitely
thin layer of a liquid is compressed by the liquid above it, and
supported by that below. Its degree of condensation depends upon the
magnitude of the compressive force; and, as this force decreases rapidly
towards the surface, where it vanishes the density of the liquid
decreases also. M. Poisson has shown that, when this force is omitted,
the capillary surface becomes plane, and that the liquid in the tube
will neither rise above nor sink below the level of that in the cup. In
estimating the forces, it is also necessary to include the variation in
the density of the capillary surface round the edges from the attraction
of the tube.
The direction of the resulting force determines the curvature of the
surface of the capillary column. In order that a liquid may be in
equilibrio, the force resulting from all the forces acting upon it must
be perpendicular to the surface. Now it appears that, as glass is more
dense than water or alcohol, the resulting force will be inclined
towards the interior side of the tube; therefore the surface of the
liquid must be more elevated at the sides of the tube than in the centre
in order to be perpendicular to it, so that it will be concave as in the
thermometer. But, as glass is less dense than mercury, the resulting
force will be inclined from the interior side of the tube (N. 174), so
that the surface of the capillary column must be more depressed at the
sides of the tube than in the centre, in order to be perpendicular to
the resulting force, and is consequently convex, as may be perceived in
the mercury of the barometer when rising. The absorption of moisture by
sponges, sugar, salt, &c., are familiar examples of capillary
attraction. Indeed the pores of sugar are so minute, that there seems to
be no limit to the ascent of the liquid. Wine is drawn up in a curve on
the interior surface of a glass; tea rises above its level on the side
of a cup; but, if the glass or cup be too full, the edges attract the
liquid downwards, and give it a rounded form. A column of liquid will
rise above or sink below its level between two plane parallel surfaces
when near to one another, according to the relative densities of the
plates and the liquid (N. 175); and the phenomena will be exactly the
same as in a cylindrical tube whose diameter is double the distance of
the plates from each other. If the two surfaces be very near to one
another, and touch each other at one of their upright edges, the liquid
will rise highest at the edges that are in contact, and will gradually
diminish in height as the surfaces become more separated. The whole
outline of the liquid column will have the form of a hyperbola. Indeed,
so universal is the action of capillarity, that solids and liquids
cannot touch one another without producing a change in the form of the
surface of the liquid.
The attractions and repulsions arising from capillarity present many
curious phenomena. If two plates of glass or metal, both of which are
either dry or wet, be partly immersed in a liquid parallel to one
another, the liquid will be raised or depressed close to their surfaces,
but will maintain its level through the rest of the space that separates
them. At such a distance they neither attract nor repel one another; but
the instant they are brought so near as to make the level part of the
liquid disappear, and the two curved parts of it meet, the two plates
will rush towards each other and remain pressed together (N. 176). If
one of the surfaces be wet and the other dry, they will repel one
another when so near as to have a curved surface of liquid between them;
but, if forced to approach a little nearer, the repulsion will be
overcome, and they will attract each other as if they were both wet or
both dry. Two balls of pith or wood floating in water, or two balls of
tin floating in mercury, attract one another as soon as they are so near
that the surface of the liquid is curved between them. Two ships in the
ocean may be brought into collision by this principle. But two balls,
one of which is wet and the other dry, repel one another as soon as the
liquid which separates them is curved at its surface. A bit of tea-leaf
is attracted by the edge of the cup if wet, and repelled when dry,
provided it be not too far from the edge and the cup moderately full; if
too full, the contrary takes place. It is probable that the rise of the
sap in vegetables is in some degree owing to capillarity.