Electric Origin of Molecular Attraction. 639 



a period 36 x 1()~ ]7 . It is worth noting that this period 

 corresponds to a frequency 28 x 10 14 which is abont ten times 

 as large as that of the visible part of the spectrum, and is 

 nearly equal to the 33 x 10 14 which in u The Cause of the 

 Structure of Spectra" (Phil. Mag. [6] ii. p. 273) was 

 found to be the value of a fundamental spectral constant 

 denoted by 1/A = VB, where V is the velocity of light in 

 free aether and B the parameter in Balmer's formula, which 

 Rydberg assumes to be a constant of nature in his modified 

 form of it, namely n = n — B/(m + /u,) 2 . 



We shall now resume the study of the Laws of Molecular 

 Force in the light of the electron theory. 



5. Electric Doublets in different classes of Chemical 

 Substances. 



Before considering in some little detail the laws of (M 2 Q*, 

 which are those of s, for different chemical types, we must 

 discuss the remarkable contrast shown (xxxv.) between the 

 characteristic equations of element and compound gases. 

 For the element gases U 2 , N 2 , 2 and also for the compound 

 CH 4 the equation of van der Waals represents the experi- 

 mental facts down to nearly two-thirds of the critical volume. 

 It can be written 



^•= RT+RT 2(^r', w 



where the terms are respectively two thirds of the following, 

 the virial of the pressure, the kinetic energy, the virial of 

 the collisional forces, and the virial of the molecular attrac- 

 tions. For compound gases in general the type is 



^m+BT^-JLj (5) 



Ethylene was found to have an intermediate form of equation, 

 and probably other substances could be investigated to show 

 different stages of transition from (4) to (5). Evidently (5) 

 could be made more general by replacing k in each of the 

 three places where it occurs by a different parameter, but, as 

 in xxxv., we will continue to use it in its more convenient 

 simple form. When we contrast the collisional virial term 

 for element gases Rlk['2(v-k/2) with RT2k/(v + k) for 

 compounds, we see that in the first case v is diminished hv 

 i/2, and in the second v is increased by k. Now, according 

 to the kinetic theory, the —k/2 comes in because the mole- 

 cular free path is shortened by an amount depending on the 



