pressure; and molecular volume. A second contribution by Van der Waals to the theoretical side of the question, in The Theory of Corresponding States, was even more important than his original essay. This theory with the propositions that have been developed has been of the greatest importance in directing experimental investigation and in attacking the difficult problem of the liquefaction of the most permanent gases. In this matter theory has far outrun experiment. In the meantime experiment had gone on, and most of the gases had been lique. hed; and the problem had become not so much how to produce intense cold as how to save it when produced from being immediately leveled up by the relatively superheated surroundings. For this purpose the author contrived a system of double-walled vessels, with the space between the walls very highly exhausted. "Vessels of this kind are now in general use, and in them liquid air has crossed the American Continent." Liquid hydrogen-that being the last of the gases yet refrigerated-is the lightest liquid known to exist, and by far the coldest. It boils at — 252.5° C., or 20.5° absolute. Reduction of the pressure by the air-pump brings down the temperature to 258°, when the liquid becomes a solid resembling frozen foam, and this by further exhaustion is cooled to 260°, or 13° absolute, the lowest steady temperature that has yet been reached. "This gap of 13° might seem at first sight insignificant in comparison with the hundreds that have already been conquered. But to win one degree low down the scale is quite a different matter from doing so at higher temperatures; in fact, to annihilate those few remaining degrees would be a far greater achievement than any so far accomplished in low-temperature research." With the liquefaction and evaporation of a gas as much more volatile than hydrogen as that is than nitrogen it might be possible to reach a lower temperature say 5° absolute but even a second hypothetical substance of like relative volatility-as perhaps helium-would not bring the experimenter quite to the point of his ambition. "That the zero will ever be reached by man is extremely improbable. . . . But supposing all difficulties to be overcome and the experimenter to be able to reach within a few degrees of the zero, it is by no means certain that he would find the near approach of the death of matter sometimes pictured. Any forecast of the phenomenon that would be seen must be based on the assumption that there is continuity between the processes studied at attainable temperatures and those which take place at still lower ones." The speaker doubted whether such an assumption was justified.

In a communication to the National Academy of Sciences, April, 1902, Prof. T. W. Richards treated of the hypotheses that may reasonably be tried to account for changes in atomic volume. It seems very extraordinary, he held, that notwithstanding the stupendous mass of chemical facts that have been collected and the important researches that have been made in physical chemistry, we are still without knowledge of the nature of a chemical compound and of the manner in which its constituents are held together. We are not even certain that they are held to gether by mutual attractions; for though the evolution of heat usual when bodies combine indicates a satisfaction of mutual forces simultaneously with the act of combination, there are still cases in which heat is absorbed during combinations. This fact suggests an inquiry whether VOL. XLII.-7 A

there may not be other agencies than mutual forces the action of which results indirectly in the formation of chemical bodies; and whether, if so, such agencies may not be a factor even of those combinations in which they are aided by direct forces. When hydrogen and chlorine come together no condensation of which account is taken occurs, although an enormous evolution of heat takes places. The extreme chemical activity of the resulting gas, however, seems to prove that it is not a complete chemical compound. Notwithstanding the great energy with which the ions have approached one another, they are still so active that the case must be regarded as very different from that when a mixture of hydrogen. and oxygen loses one-third of its volume in combining. A body may undergo contraction without combining with a different body, as in the case when the vapor of water is cooled. In all such cases heat is plenteously evolved, and molecule unites with molecule. It may be questioned how far the heat of chemical reaction is anything more than heat of contraction. Contraction does not necessarily consist solely in the approach of atoms toward one another. If an atom is a vortex, it must be in itself elastic and capable of deformation; and it may be so even if it is not a vortex. If atoms are compressible the mutual attraction between two atoms would naturally tend to deform them. Hence we have, further, to determine the relation between such deformations and their valency. Apparently, highly compressible elements have low valency, while elements of high valency, like carbon, are little compressible. Such considerations as these noted should be borne in mind in the construction of new hypotheses upon which experimental investigations are to be based.

In his presidential address before the section of chemistry of the British Association Prof. Edward Divers, of the Imperial University of Tokio, Japan, presented what he designated as The Atomic Theory without Hypothesis, or divested of the conception of atoms as discrete particles. He thought that the conception of bodies as not continuous in texture, but as composed of discrete minute particles, had been a bar to the full and universal acceptance of Dalton's atomic theory, and offered a new view, divested of all reference to the physical constitution of matter, in which only the conditions of chemical equality between substances should be regarded.

In their study of the theory of radio-activity, E. Rutherford and Frederick Solly observe that all the most prominent workers in the investigation, including M. and Mme. Curie and M. Becquerel, agree that the phenomenon is the function of the atom and not of the molecule. In the experiments of the authors with the emanation produced by thorium compounds and the property it possesses of exciting radio-activity on surrounding objects the radio-activity appeared in each case as the manifestation of a special kind of matter in minute amount. The emanation behaved in all respects like a gas, and the excited radio-activity it produced as an invisible deposit of intensely active material, independent of the nature of the substance on which it was deposited, and capable of being removed by rubbing or the action of acids. The position is thus arrived at, that radio-activity is at once atomic phenomenon and the accompaniment of a chemical change in which new kinds of matter are produced. Before such a view was entertained attempts made to explain it on existing

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