MOLECULE 611 composed of these distinct portions of matter separated, or capable of being separated, by empty space from other portions. But the molecular hypothesis of the constitution of different kinds of substances aims at analysing this process by which such substances are built up out of their constituent atoms. The molecule of any substance is, by some chemists, defined as being the smallest portion of that substance to which can be attributed all the chemical pro perties of the substance ; by others, as the smallest portion which, so long as the substance is chemically unchanged, keeps together without complete separation of its parts. In the language of Clausius s theorem, if the parts of the molecule have internal motion, the kinetic energy of such internal motion is equal to the virial of the mutual attrac tive forces of the parts. Thus the formation of the mole cule of each particular substance is viewed as an essential step in the process of building up that substance out of its constituent atoms. The molecule is first built up out of atoms arranged in its formation according to a definite type, and then the substance itself is constituted of these molecules. Of course molecules may be, and in fact in many particular substances are, supposed to be mon- atomic ; that is to say, the intermediate step of building up the molecule out of the atoms has, in these particular substances, been omitted, the atoms and molecules becom ing then identical. The particular arrangement of the formed molecules in the building up of the substance de termines the physical state of that substance, that is, its fluid, solid, gaseous, crystalline, or amorphous state ; but the chemical properties of the substance depend upon the constitution of the molecule. As the investigations and theories of chemistry appear to indicate irresistibly the existence of permanent atoms, so do they also lead almost as necessarily to the conception of the molecule as an entity which bears the same relation to special substances that the atoms bear to matter generally. So long as the molecule endures, the substance of which it is the molecule retains its chemical properties ; with the dissolution of the molecule, the substance, as that special substance, perishes ; the atoms alone continue, and are free to enter into other combinations. The permanence of the molecule is relative, that of the atom absolute. This con ception of the molecular constitution of substances sug gests physical questions of great interest, such as the shape, volume, and mass of the constituent molecules, and the relative motions of which their parts are susceptible ; and the answers to these questions cannot fail to be of great value in chemical and chemico-physical investigations, as well as in the theories of light and electricity. Now, whatever differences may exist between the proper ties of different substances in the solid and liquid states, there are certain properties which, in the gaseous state, manifest themselves with no variation whatever in all sub stances alike. Hence the explanation of these common properties or gaseous laws, as they are called has long possessed a peculiar fascination for physicists. The tend ency to expand or fill all accessible space, manifested by all gases, proves that on the molecular hypothesis their compound atoms or molecules must be continually tending to fly apart. We must conceive gases as constituted of mole cules, not only separable but actually separated by space void of the matter of which these gases consist ; and it may be most reasonably expected, therefore, that any general laws to which substances in this state conform may afford us a valuable insight into the constitution of these separate molecules. Now the general laws to which all gases conform are : (1) Boyle s law that, in a given mass of any gas kept at constant temperature, the pressure per unit of area upon the containing surface increases in the same proportion as the volume occupied by the gas is diminished, or at least with very slight deviation from exact proportionality ; (2) Charles s law that, if the temperature be varied while the pressure upon the gas remains the same, the gas increases by Trad of its volume at zero centigrade for every degree of centigrade added to the temperature, or, which in com bination with Boyle s law is the same thing, that if the density be constant, the pressure is directly proportional to the temperature measured from the point - 273 centigrade, this point being called the zero of absolute temperature ; (3) Avogadro s law which asserts that all gases at the same temperature and pressure contain the same number of molecules in the same volume ; and (4) Dalton s /aw that in a mixture of different gases, when there is equilibrium, each gas behaves as a vacuum to all the rest. It was at one time considered that these phenomena might be explained on the hypothesis of mutual repulsive forces between the parts of which the gas is composed, whether they were regarded as constituted of molecules or of infinitely divisible continuous matter, 1 but it has been shown in the article ATOM (vol. iii. p. 39 sq.) that there are at least two absolutely conclusive reasons why this ex planation cannot be accepted. These objections, together with the experimental fact proved by Joule that gases, or at any rate atmospheric air, expand into vacuum with scarcely any appreciable change of temperature, must be considered fatal to any mutual-force theory of gaseous action, and, accordingly, physicists have been driven to seek for other methods of explaining these laws. The explanation which has been more developed than any other is that known as the kinetic theory of gases, which regards the intrinsic energy of a gaseous mass as residing, not in the potential energy of intermolecular forces, but mainly in the kinetic energy of the molecules themselves, which are assumed to be in a state of continual relative velocity, admitting at the same time a possible small intermolecular potential energy, and it may be also an interatomic energy, between the atoms of the individual molecules. That some such persistent relative motion does exist in every gaseous mass is evident from the rapidity with which odours penetrate the stillest air where no breath of wind that is, of absolute motion of translation of the mass as a whole or any portion of finite size is perceptible. It becomes an interesting question whether the laws of mechanics admit of a mass thus constituted ever arriving at a state of permanence ; that is to say, whether, consistently with the hypothesis of infinite irregularities in the directions and magnitudes of velocities of individual molecules, there may be found any properties of the mass in the aggregate which remain 1 An argument in favour of the molecular constitution of gases, to which attention was first called by Professor Osborne Reynolds (Memoir " On some Dimensional Properties of Matter in the Gaseous State," Phil. Trans., 1879), is derived from certain phenomena observed in highly-rarefied gases, and in the transpiration of gases through porous plates. If, according to this argument, we had in a gas to do with a continuous plenum, such that every portion must possess the same properties, then these properties must exist inde pendently of the amount of gas contained in any space, although their sensible effects might be increased or diminished by a variation in that amount. If, then, we can find properties of a gas depending on the size of the space in which it is enclosed, and on the quantity of gas enclosed in this space, we have proof that gas is not continuous in other words, possesses dimensional structure. Such properties we do find in highly-rarefied gases, as, for instance, in the pheno mena of Crooke s radiometer. The motion of the vanes when one side is heated by incident rays appears to depend on the distance between the vane and the containing walls of the vessel bearing some not very high ratio to the distance between the particles or molecules of the gas. At least no satisfactory explanation of the phenomena consistent with the gas being continuous has yet been suggested. Again, Professor 0. Reynolds, from his experiments on the trans piration of gases through a porous plate, finds a relation between the gas and the coarseness or fineness of the plate, which would not exist
were the gas continuous.