466 



The author thinks that the unequal differences in boiling-points 

 corresponding in different homologous series to the elementary differ- 

 ence #C 2 H 2 , are probably regulated by a more general law, which will 

 be found when the boiling-points of many substances shall have been 

 determined under pressures differing from those of the atmosphere. 



"From the observations at present at our disposal it may be 

 affirmed as a general rule, that in homologous compounds belonging 

 to the same series, the differences in boiling-points are proportional 

 to the differences in the formulae. Exceptions obtain only in cases 

 when terms of a particular group are rather difficult to prepare, or 

 when the substances boil at a very high temperature, at which the 

 observations now at our command are for the most part uncertain. 

 Again, it may be affirmed that the difference in boiling-points, 

 corresponding to the elementary difference C 2 H 2 , is in a great many 

 series =19 ; in some series greater, in some series less." 



The author proceeds to discuss the boiling-points of isomeric com- 

 pounds. He shows that in a great many cases isomeric compounds 

 belonging to the same type, and exhibiting the same chemical cha- 

 racter, boil at the same temperature, and that there is no reason why, 

 for the class of bodies mentioned, this coincidence should not obtain 

 generally. On the other hand, different boiling-points are observed 

 in isomeric compounds possessing a different chemical character, 

 although belonging to the same type (e. g. acids and compound ethers, 

 C n H n O<i ; alcohols and ethers, C w H ra+2 O 2 ), and in isomeric com- 

 pounds belonging to different types (e.g. allylic alcohol and acetone). 



The author shows that the determination of the boiling-point of a 

 substance, together with an inquiry into the compounds serially 

 allied with it by their boiling-points, constitutes a valuable means of 

 fixing the character of the substance, the type to which it belongs, 

 and the series of homologous bodies of which it is a term. He 

 quotes as an illustration eugenic acid. The boiling-point of this 

 acid, C 20 H 12 O 4 , is 150 ; and on comparing this boiling-point with the 

 boiling-points of benzoic acid, C 14 H 6 O 4 (boiling-point 253), and of 

 hydride of salicyl, C U H 6 O 4 (boiling-point 196), it is obvious that 

 eugenic acid cannot be homologous to benzoic acid, whilst, on the 

 other hand, it becomes extremely probable that it is homologous to 

 hydride of salicyl, and consequently that it belongs rather to the 

 aldehydes than to the acids proper. 



