PHYSICS. 589 



and found that they remained constant until coalescence was produced, 

 and then disappeared suddenly; thus eliminating conclusively the hy- 

 pothesis that the action of electricity in promoting union may be ascribed 

 to the additional pressure called into play by electrical attraction of the 

 opposed water surfaces acting as plates of a condenser. {Phil. Mag.y 

 July, 1885, V, XX, 31.) 



3, Of Gases. 



An interesting discussion on the kinetic theory of gases took place in 

 the mathematical and physical section of the British Association at the 

 Aberdeen meeting in September. The discussion was opened by Crum 

 Brown, who stated the difficulties of the theory under two heads : First 

 the diflicuUies connected with the doctrine that energy communicated 

 from without to a gas is equally shared among the whole of the degrees 

 of freedom of the molecules ; and, second, the difficulties connected with 

 the doctriDe that energy of each kind is distributed among the mole- 

 cules according to some form of the law of probability. Under the first 

 head he called attention to the fact that the ratio of the specific heat of 

 mercury vapor at constant pressure to that at constant volume is 5 : 3, 

 which gives on the dynamical theory but three degrees of freedom to the 

 molecules, and these must be the three translational freedoms. If to 

 prevent rotation the molecules be regarded as perfectly smooth, rigid, 

 and spherical, the radiation producing the spectrum cannot be accounted 

 for. In diatomic gases the ratio is 7 : 5, giving three translatory and 

 two rotational freedoms. But here again the vibration of the atoms 

 is not accounted for, either as parts of the molecule or individually. 

 Boltzmanu's theorem asserts that the energy of a molecule is equally 

 distributed among the difi'ereut degrees of freedom. Hence, if in addi- 

 tion to the six degrees of freedom of a rigid body in space the mole- 

 cules have twenty or thirty others, it would seem that the dynamical 

 theory must be abandoned, as there would not be sufficient energy for 

 translational motion. The second class of difticulties arises when it is 

 supposed that the energy is distributed among the molecules according 

 to some form of the law of probability. For then in a mixture of gases 

 there would always be some molecules in a condition favorable for com- 

 bination. Moreover, there should be no such sharp temperature and 

 pressure limits for combination, such as exist between phosphorus and 

 oxygen, for example. And further, oxygen and hydrogen may be kept 

 for a long time at a temperature near that of combination without any 

 union taking place. Li veing maintained that the difficulties encountered 

 arose from the assumptions of Boltzmann's theorem, which is not a neces- 

 sary part of the kinetic theory. It is quite possible that mercury vapor 

 has no sensible vibrational energy at the temi^eratures at which its spe- 

 cific heat has been measured. The more perfect gases have at ordinary 

 temperatures much less vibrational than translational energy; so that 

 they may have only one or at most two modes of vil)ration. Sir Willianj 



