106 EEPORT— 1891. 



very nearly the same, critical pressure ; so that at any equal 

 pressure {e.g., 760mra.) they are at equal fractions of their 

 critical pressure, and are truly comparable. From this and 

 other facts proved by Young, and from the general similarity 

 between his group of compounds and the other groups con- 

 cerned, I drew the conclusion that, in all inohability, equality 



. V . 



of the ratio fy^in different compounds at eqtial pressure depends 



on a covihination of the following : (1) similarity of chemical 

 constitution of a kind ahin to that concerned in isomorphism, 

 (2) equality of critical pressure ; and implies also (3) constancy 



V T 



of the ratios yj and m7 at all equal pressures . 



My publication (Phil. Mag., Nov., 1890) of the paper of 

 which I have here given a resume induced Professor Young to 

 publish (same journal and date) a preliminary account of the 

 conclusions he had drawn from a hypothesis of van der Waals 

 as tested by work of his own. These conclusions, which he 

 only puts forward tentatively in the meantime, are three in 

 number, and relate to the interconnection of volume, tem- 

 perature, and critical pressure in the case of liquids. His first 

 conclusion is practically identical with that which I have just 

 quoted from my own paper. The second deals with the case of 

 chemically-similar compounds of unequal critical pressure, in- 

 cluding such cases as that of CCI4 and SnCl4, already referred 

 to, and shows that these are comparable at equal fractions of 

 their critical pressures, but not at equal pressures. The third 

 conclusion refers to compounds which are not chemically 

 similar and have not equal critical pressures, in which case 

 no simple law can be at present indicated. 



By observations on volumes and boiling-points at atmo- 

 spheric pressure it is unlikely that any generalisations of 

 interest, beyond those that I have already indicated, will be 

 discovered. Professor Young, who is experimenting on liquids 

 up to their critical points, is on the only safe path to the proper 

 elucidation of the connection between pressure, temperature, 

 and volume, which are the most important physical charac- 

 ters of a liquid, as they are of a gas. I for one shall await with 

 great interest the further developments of his work. In the 

 meantime, I may point out that the generalisation which I 

 ventured to make, and which Professor Young independently 

 arrives at, may be serviceable in this way : If a number of 



V 



chemically-similar bodies give an equal value for -= at atmo- 

 spheric pressure, and if the physical constants of only one of 

 these bodies be known, those of the others can be provision- 

 ally and approximately calculated. 



