THE KINETIC THEORY OF MATTER. 155 



pV 2 

 Putting for p its approximate value ?-- in the small quantity pvr we get 



9 



6m 

 a 



I 



where v = - and a = - 



p om 



and the equation becomes 



The assumption that a is constant implies that V 3 7rs 2 T is constant. We 

 have no data to justify this, and herein lies a weakness in the investi- 

 gation. 



It may be mentioned that Van der Waals has calculated b from 

 observations on the compression of gases. This gives another method of 

 finding s, and the results are of the same order as those already found. 



The equation of Van der Waals has been modified in various ways 

 to make it represent the facts of observation more closely. But the 

 modifications are probably suggested rather by the lack of accordance 

 with fact than by the kinetic theory, and must be regarded as somewhat 

 empirical. For an account of some of these modified equations we refer 

 the reader to Meyer's Kinetic Theory of Gases, p. 100. Another modifi- 

 cation has been proposed by Callendar (Proc. R. S., Ixvii., 1900, p. 266). 

 We shall consider the equation of Van der Waals further in chap. xi. 



We have already seen that some of the energy given to a gas on 

 raising its temperature goes, in all probability, to increase the internal 

 energy of the molecule. This brings us to the point where the theory 

 for the present stops. No satisfactory hypothesis as to the construction 

 of the molecules, and the mode in which they possess energy, has as yet 

 been devised. 



In order to find out what goes on in a molecule we should, of course, 

 like to isolate and study it in the method by which alone we could do so, 

 viz., by examining the nature of the energy it gives out. But this is 

 impossible. The nearest approach to isolation is in a rare gas, where 

 the molecules have a long free path, so that, for a great part of their 

 time, they are not interfering with one another. When such a gas is 

 heated under certain conditions, it sends out waves of light of definite 

 refrangibilities, showing that in all probability the molecules or their 

 parts are vibrating, but we are quite unable to do more than guess at the 

 mode of vibration giving rise to these light waves. It may be that the 

 different atoms in the molecule are vibrating about the common centre 

 of gravity, as the earth and moon vibrate about their common centre of 

 gravity ; or, it may be that the separate atoms vibrate as a bell vibrates. 

 If the latter is the mode of vibration giving rise to light-waves, then we 

 must suppose that the contiguity of other dissimilar atoms alters the 

 character of the vibrations, for we know that the light sent out by 

 incandescent compounds is different in character from that sent out by 



