72 THE TRUE VALUE OK a OF VAN DE It WAALS 1 



stances whether we compute a from the critical data, at a tempe- 

 rature of from 200° to 300° C, or from the surface tension 

 determinations made at temperatures even as low as zero or in some 

 cases 20° — 30° below zero. 



The constancy of a is thus established, in my opinion. If asso- 

 ciation or dissociation occur with a rise of temperature, then indeed 

 a will not be constant, since all the factors which are included in 

 a change if association takes place, namely, the number of the 

 molecules, or N, the molecular weight and the number of valences. 



VI. THE RELATION OF COHESION TO MAGNETISM. 



What is the significance of the fact that the cohesion depends upon 

 the valence electrons and that without valence there will be no cohesion ? 

 Can the significance be anything else than that the cohesion is electro- 

 magnetic and that in consequence cohesion is closely related to magne- 

 tism? Or rather that perhaps magnetism is only cohesion made apparent 

 in certain substances at more than molecular distances? I can see no 

 other interpretation of these facts than this, and I have in an earlier 

 paper pointed out some of the reasons which have seemed to me 

 to point clearly in this direction. Like Sutherland I hesitated for 

 some time whether the significance is that cohesion is electro-static 

 or electro-magnetic, but the peculiar and exceptional position of 

 oxygen, carbon monoxide and dioxide and one or two other sub- 

 stances, seems to me to indicate with entire defmiteness that what 

 I have called valence is the number of negative electrons. If this 

 is the case there can hardly be a question here of electro-static 

 attraction, since the cohesion is due in all cases to the negative 

 electrons. These will only attract electro-magnetically and not 

 electro-statically. I have come to the opinion, therefore, that the 

 cohesional attraction is indeed magnetic in nature and that the 

 cohesive mass is electro-nlagnetic just as is the gravitational mass. 

 We see indeed the remarkable resemblance between magnetism and 

 cohesion. Each extends ordinarily but a single molecular diameter 

 for the reason that the orientation of the molecules is haphazard. 

 They neutralize each other's magnetic fields or cohesive iields. So 

 we can discover magnetism and cohesion in most substances only 

 in the immediate neighborhood of the molecules. It is only in a 

 few substances that the molecules or atoms are of such a nature 

 that they do not neutralize each other's magnetic fields, but they 

 reinforce them and in these substances the attraction may be easily 

 perceived at distances more than molecular. 



