a stone wall or a wooden door becomes as permeable as plate glass to sunshine. All this has long been suspected by the physicists who, among equally significant facts, have noticed that an explosive is set off less by the violence of its detonator than by the sympathy of rhythm between the two. Dr. Lothar Meyer, in his Modern Theories of Chemistry, discusses the ingenious theories which on kinetic principles explain many of the chief qualities of matter—color, refrangibility, volatility, fusibility, and ability to yield heat in combustion. He regards this field as that which bears most promise for the chemical investigator, and follows Berthollet in maintaining that chemistry is but a branch of the larger science of mechanics. In corroboration of this view a thousand facts might be cited—a typical piece of evidence is that adduced by Mr. Witt, who finds that the stability of the azo-benzene dyes turns upon the nicety with which their acid and basic functions balance each other.
In leaving the field of molar for that of molecular mechanics, it has been already noted that friction need no longer be reckoned with; consequences equally important result from the fact that now masses of extreme minuteness are in play. Sir William Thomson (Nature, vol. i. p. 551) has estimated the diameter of molecules as at most 1760000000 of an inch in diameter; cubical molecules of this size containable in a cubic inch of space would have a total surface of one square mile and one seventh, which implies that in molecular mechanics superficial forces must count for vastly more than in molar mechanics. Another result follows from molecular minuteness of dimensions—an enormously increased capacity for motion. The smaller a wheel the more swiftly can it be rotated without being parted by centrifugal force, and therefore the more motion can it contain. With a molecule probably, with an atom certainly, centrifugal force has no separating power. How great the momentum of specific molecular motions will appear in computing that due to temperature, in the case of a pound of unfrozen water at the zero of the centigrade scale. According to the determinations of Lord Rayleigh, a pound of water, in falling through one degree of temperature, liberates heat equal to that generated were the mass to fall from a height of fourteen hundred and two feet to the surface of the earth. Therefore, in first becoming ice, and then falling in temperature through two hundred and seventy-three degrees, it parts with an amount of energy equal to lifting the pound of water some fifty-seven and one third miles from the surface of the earth, leaving out of view, for simplicity's sake, the diminution in the attraction of the earth as the mass is lifted. Prof. Dewar, in his recent remarkable experiments at a temperature of 200º below zero, has found reason to believe that at absolute zero the electri-
