40 Prof. Liveing, On the variation of intensity of [Nov. 28, 



repeated a good many times, but it did not, to any sensible degree, 

 bring about a combination of the gases, for on opening the tube 

 in a solution of soda the chlorine was absorbed and the hydrogen 

 left in the original volume. 



I think, then, that it must be allowed that, though at the 

 temperature of the compressed gas in the sound-wave no sensible 

 fraction of the heat is converted into vibratory motion in the 

 molecules of mercury, this does not hold for higher temperatures. 



If this be admitted we must abandon the hypothesis that, in 

 gases, energy communicated to the molecules is distributed equally 

 in all the degrees of freedom. I could never see any valid reason 

 for this hypothesis, and many physicists have long ago repu- 

 diated it. 



But there is another point. Some people see in the ratio of 

 the specific heats of mercury vapour an argument for supposing 

 that the molecules of that vapour, which are chemical atoms, are 

 really the rigid spheres, which the old Epicurean philosophy 

 suggested, and which have become familiar by their use as a 

 working hypothesis for facilitating the mathematical calculations 

 of the kinetic theory of gases. This hypothesis is untenable for 

 other reasons besides the facts here adduced. Nevertheless it 

 underlies the assumption that monatomic molecules are incapable 

 of acquiring any vibratory or internal form of motion from energy 

 communicated to them as heat, whence it has been concluded 

 that every gas which is found to have 1"66, or thereabouts, for the 

 ratio of its specific heats must be monatomic. It is possible that 

 a chemically monatomic molecule may have, though it is not 

 probable that it really has, a simpler constitution than a chemi- 

 cally complex molecule, and so may have not so many degrees of 

 freedom as the latter, but still a plurality of degrees. 



(2) On the variation of intensity of the Absorption Bands of 

 different Didymium Salts dissolved in water, and its bearing on the 

 ionization theory of the colour of solutions of salts. By Professor 

 Liveing. 



In common with many others I have spent much time and 

 trouble in trying to separate the elements of the Yttrium and 

 Cerium groups of earths. Latterly my assistant, Mr Purvis, has 

 for two or three years been fractionating some mixtures of these 

 earths, and, though he has not succeeded in getting products 

 spectroscopically pure, he has come across results which appeared 

 to me worth following up. The investigation is yet far from 

 complete, but a preliminary account of it may be of some interest. 



The main facts are that while the absorption bands produced 

 by solutions of the chloride and the nitrate have the same positions 



