EQUATION AND THE NATURE OP COHESION. 63 



extreme cases. But it is significant that excepting the oxygen coin- 

 pounds of doubtful valence, these deviations occur in those com- 

 pounds, namely iodobenzene and brom benzene, in which the critical 

 data are most uncertain. We have the best agreement in benzene, 

 isopentane and pentane, substances of which all the data are most 

 accurately known. 



It is clear that the law of the dependence of the mass factor of 

 cohesion on the gravitational mass and valence is substantiated. 

 The factor a of van der Waals' equation may be computed from 

 the molecular weight and number of valences, if these be known, 

 as accurately, or indeed more accurately, than in any other way. 

 Cohesional attraction is thus shown to be gravitational attraction 

 intensified in the immediate vicinity of the molecules by the fiction 

 of the valence electrons. That cohesion should be of this nature is 

 very probable. It is certain that a has in it the factor N 2 , N being 

 the number of molecules in the mass of liquid or gas for which a 

 is given. This follows from the fact that a varies as the square 

 of the volume of the fluid taken and hence as the square of 

 the number of molecules. That the molecular weight is one 

 of the factors in a has been believed by most students, for a 

 advances steadily in homologous compounds as the molecular weight 

 advances. It is the number of valences which is the other factor, 

 the importance of which I have discovered, altho Sutherland, 

 before me, suggested that possibly the valences were of importance; 

 but he could not prove it and he afterwards thought that the 

 gravitational mass played no part in cohesion. In this he was 

 wrong. 



It seems to me that the facts that I have adduced suffice to 

 prove the truth of the law I have formulated. 1 trust that the 

 additional evidence which I have thus been able to adduce as to 

 the true value of a may remove all doubts as to the validity of a 

 law which so simply and so naturally correlates such puzzling phe- 

 nomena and which enables us to predict the cohesion of any sub- 

 stance if only we know the molecular weight and the number of 

 valences. Or if we know the cohesion which enables us to deter- 

 mine the number of valences in the molecule, a matter of the 

 greatest importance in the study of the laws of refraction in the 

 study of magnetism and of chemical constitution. It has besides the 

 very great advantage of enabling us to calculate a with at least 

 approximate certainty, so that we can proceed to calculate b, the 

 other constant which expresses the volume of the molecules, and we 

 may discover how it varies with temperature and pressure. When 



