Our knowledge of the constitution of the molecule, although still meagre, has been ex traordinarily advanced in the last few years. In the earlier scientific treatment of the molecular theory, atoms and molecules were assumed to be hard, smooth, spherical and perfectly elas tic. It is doubtful if any competent physicist really believed them to have these characteris tics, but the difficulties involved in the mathe matical treatment of the molecular theory were considered to be insuperable unless as sumptions of the kind here indicated were made. It should he understood, however, that even although the actual molecules or atoms, if we could examine them individually and closely, might not resemble, in the least degree, the description of them thus given, yet they might behave collectively in much the same way as the spherical particles to which the mathematician found it necessary to confine his first investigations. Moreover, certain characteristics of the molecule may sometimes be accounted for by means of one set of as sumptions, while certain other characteristics are best explained by the aid of an entirely different set. It is, therefore, wise to con sider all the different constitutions that sug gest themselves, in order to see what particu lar phenomena each one harmonizes with; for by doing this we may perhaps be aided in our approach to the final solution of the problem.
Rankine, following certain suggestions of Sir Humphrey Davy, developed a theory of molecular structure which he called the ((hy pothesis of molecular vortices" and which as sumed that each fundamental atom consists of a nucleus or central point surrounded by an elastic atmosphere which is retained in posi tion by attractive forces. He ascribed the elasticity of vapors to the centrifugal force de veloped in these atmospheres, which he as sumed to be in a state of rapid revolution or oscillation about their central nuclei, and he believed that light is propagated through mat ter by means of the vibration of the central atomic nuclei, which were assumed to exert attractive or repulsive forces upon one an other. (To account for the propagation of light through free space he assumed a certain special constitution for the ether, with which we are not at present concerned). From this hypothesis of molecular vortices Rankine de duced a number of the known properties of bodies, being Specially successful in connection with the pressure of saturated vapors; but sub sequent mathematicians and physicists did not find the hypothesis attractive or promising and his theory still remains in the condition in which he left it, although it may yet be found to be a sort of crude forerunner of the modern corpuscular theory of molecular constitution.
Lord Kelvin made the interesting sugges tion that atoms are merely definite portions of the ether of space, distinguished from the re mainder of the ether by being endowed with a peculiar kind of motion called uvortex mo tion." The smoke rings that are sometimes seen over the stacks of locomotives afford familiar examples of vortex motion and the cyclone is another example on a grand scale.
According to the principles of hydrodynamics it appears thata vortex ring, once set in motion in a frictionless fluid, would be in destructible and eternal. Moreover, it would possess certain elastic properties that are of interest, nterest, Inasmuch as perfect elasticity is one of the postulates that physicists have demanded for the atom or molecule from the very earliest times. It will be observed that the vortex theory of Kelvin does away with matter as distinct from the ether of space— the atom or molecule being distinguished from the remaining portion of the ether merely by reason of the special kind of motion with which it is endowed. Interesting as Kelvin's vortex theory is, it presents numerous diffi culties that are almost insuperable as soon as we endeavor to develop it sufficiently to make it harmonize with the vast range of physical and chemical facts that we now possess.
Dr. C. V. Burton suggested, some years ago, that the atom may be simply a region in which the ether is subject to a permanent strain of some kind. This hypothesis may be capable of considerable further development, but it does not appear to harmonize with the direc tion in which present thought is tending. Dr. Burton's conception has the apparently unique merit of making it possible even to explain the origin of matter; for if the ether had long ago possessed motion of the most general kind, we might imagine its present condition to be due to the degeneration of that motion into a fine-grained turbulence, and if the con stitution of the ether is such that strain-figures of the sort imagined by Burton are possible in it, then atoms may have resulted from time to time whenever the ether chanced to become stressed, locally, to such an extent as to take what would correspond in the world of fa miliar experience to a permanent set. In the vortex theory of Kelvin, an atom always con sists of the same portion of ether, but in Burton's ether-strain theory it is only the strain that is propagated through the ether when the atom moves, the ether itself re maining motionless.
At the present time physicists strongly favor the electrical theory of the constitution of matter. According to this view, an atom is a system composed largely or perhaps wholly of discrete charges of electricity, which main tain their individuality throughout all the changes that the atom undergoes. Much more is known about the negative charges than about the positive ones, because negative charges can easily be detached from atoms and molecules and subjected to study. The little charges or masses of electricity with which we are con cerned in the investigation of molecular struct ure are commonly called °electrons," but some of the best modern authorities (following the original suggestion of J. J. Thomson) prefer to speak of them as °corpuscles" when con sidering them as physical bodies and to use the word °electron" as the name of a unit of electricity— the quantity of electricity, namely, that each corpuscle bears or of which it perhaps wholly consists. (See ELECTRON).