Page:The American Cyclopædia (1879) Volume XI.djvu/726

 708 MOLECULE implies more than the molecules integrantes of the French writers just referred to. The mole- cules of a substance are not merely small par- ticles of that substance, but they are isolated masses, or, to use the words of Sir William Thomson, " pieces of matter of measurable di- mensions, with shape, motion, and laws of ac- tion, intelligible subjects of scientific investiga- tion." The term therefore involves the con- ception that the molecules of a substance are as definite magnitudes as the stars, and that every mass of matter is a collection of such bodies, just as a stellar cluster is a collection of suns. The molecules of any one substance, how- ever, are supposed to be alike in all respects. There are many phenomena, both of physics and chemistry, which indicate that this concep- tion is just and accurate. We will only refer to two of the most familiar. When by boiling under the atmospheric pressure water changes into steam, it expands, as compared with its volume at the point of maximum density, 1,700 times; or in other words, one cubic inch of water yields nearly one cubic foot of steam. Two suppositions are possible as modes of explaining this change. The first is, that in expanding the material of the water becomes diffused through the cubic foot so as to fill the space with the substance we call water as completely as before, and leaving no space within the cubic foot which does not contain its proper proportion of water. The second is, that the cubic inch of water consists of a certain number of isolated particles, the cubic foot of steam containing the same parti- cles as the cubic inch of water, and the con- version of the one into the other depending simply on the action of heat in separating these particles to a greater distance. Hence the steam is not absolutely homogeneous ; for, if we consider spaces sufficiently minute, we can distinguish between such as contain a parti- cle of water and those which lie between the particles. These assumed particles, which are thus supposed to be separated by the heat, are the molecules of the water ; and the molecular theory of the constitution of matter explains the change of volume in the manner last described. That this is probably the true explanation will be evident from a consideration of the familiar phenomena which appear when by pressure we condense steam back to water. Conceive of a cylinder filled with rarefied steam at some tem- perature above the boiling point of water. If into such a cylinder we press a piston, the volume of the steam will be diminished in pro- portion to the pressure, according to the well known law of Mariotte, up to a certain limit ; but as we increase the pressure a point will be reached, sooner or later, at which this law of compression ceases abruptly, and the steam without any intermediate transition takes a volume many hundred times less than before, changing of course into liquid water. Now, if there was a perfect continuity in the steam, we cannot conceive why there should not be a similar continuity in the law of expansion ; and on the other hand, this sudden break is perfectly explained if we are really crowding together a mass of impenetrable particles, and the whole order of the phenomena suggests this conception. Again, if the space occupied by a mass of steam is really packed close with the material we call water, if there is no break in the continuity of this aqueous mass, we should expect that the vapor would fill the space to the exclusion of everything else, or, at least, would fill it with a certain degree of energy which must be overcome before any other vapor could be forced in. But the facts are the very reverse of this. Conceive of two globes at some temperature above the boiling point of water, one filled with steam, the other completely exhausted. Let these globes be so arranged that we can introduce into each the same quantity of alcohol, and we shall find not only that the alcohol will evaporate in both, but that just as much alcohol vapor will form in the globe filled with steam as in the vacuous space, and will exert precisely the same pres- sure against the sides of the two vessels. The presence of the steam does not interfere in the least degree with the expansion of liquid alco- hol into alcohol vapor. The only difference which we observe is that the alcohol expands more slowly into the aqueous vapor than it does into a vacuum. The final result however is the same in both cases, and thus we may have two different vapors filling the same space without interfering with each other ; and more than this, so far as we know, any number of vapors, which do not act chemically on each other, may occupy the same space at the same time, each preserving its individuality so com- pletely that its relations would not be essen- tially altered if the associated materials were removed. Evidently then no vapor complete- ly fills the space which it occupies, although equally distributed through it ; and we can give no satisfactory explanation of the phenomena of evaporation except on the assumption that each substance is an aggregate of particles or units, which by the action of heat become so widely separated that they leave very large interstices in which the particles of an almost indefinite number of other vapors can find room. A study of the phenomena of evapora- tion leads to a definition of molecules, which, although not comprehensive, is for the cases it covers the most precise that can be given : Molecules are those small particles of a sub- stance which are not subdivided when the body is expanded by heat, and which move as units under the influence of this agent. As the above statement implies, the modern mo- lecular theory assumes, not only that the mo- lecules are isolated masses, but also that they are in active motion, and the phenomena of heat are regarded as manifestations of this motion. The idea that the ultimate particles of matter are in motion is as old as Democritus, but this idea was never precisely formulated