TKANSACTIONS OF SECTION B. 103 



(2.) These two liquids become one homogeneous solution 

 at a temperature which depends on the ratio of the original 

 ingredients. There is one ratio which demands a higher 

 temperature than any other. This is the critical solution- 

 temperature, above which either ingredient is infinitely soluble 

 in the other. 



1. On Molecular Volumes and Boiling-points in relation to 

 Chemical Character. 



By Okme Masson, M.A., D.Sc. 



A VERY large number of determinations of molecular volumes 

 of liquid compounds at their ordinary boiling-points has been 

 made by various observers since Kopp proclaimed his well- 

 known law. According to this law, the molecular volume of 

 a compound is the sum of the atomic volumes of its constituent 

 atoms. Thus every element has its atomic-volume value, 

 which it carries with it into its compounds. In this sense the 

 property is an additive one ; but it is also to some extent a 

 constitutive property, since the facts require that one or two of 

 the elements, at least, should be credited with alternative 

 atomic volumes and a power of entering into combination 

 sometimes with one value and sometimes with another. Later 

 researches proved that isomeric substances have not exactly 

 equal molecular volumes, as they should have if it were entirely 

 an additive property, that the constitutive feature is of general 

 importance, and that it is wrong to regard the atomic volume 

 of an element, as determined by a study of its compounds, as 

 more than a fair approximation to an average value. At a 

 definite boiling-point the molecular volume of each liquid com- 

 pound is a definite quantity ; but any attempt to divide this 

 volume among the constituent atoms can only lead to approxi- 

 mately true results. There is, however, evidence that the 

 atomic volume of an element in the free state (when it is in the 

 liquid condition at its boiling-point under atmospheric pressure) 

 is approximately equal to its average atomic volume in com- 

 pounds, or to the smaller of its volumes when it possesses two. 

 This has been shown to hold good of bromine, phosphorus, 

 sulphur, and qr ite lately of chlorine also. 



Valuable w> :k has thus been done by attempts to analyse 

 molecular voluiiies and so study the contributing volumes of 

 the elements as influenced by their modes of combination, or 

 (reversing the problem) by attempts to study the modes of com- 

 bination by assuming definite contributing volumes to charac- 

 terize the elements when combined in certain ways. But 

 comparatively few attempts have been made to study the 



