16 REPORT 1859. 



glucose and inannitane, have been shown by Berthelot* to have the function 

 of polyatomic alcohols, and are probably more than triatomic. Of the latter 

 class, meconic acid, C 7 H' O 7 , is an example: it may be regarded as deriving 

 from an unknown triatomic alcohol, C 7 H 12 O 3 (w=7, #=2), by the substitu- 

 tion of O* for H 8 . 



Comparing together the corresponding compounds of different groups, 

 chemists have long been accustomed to arrange them in homologous^ series, 

 or series in which there is a common difference between any two neighbour- 

 ing terms of CH 2 . The discoveries of late years make it appear probable 

 that series of similar compounds also exist in which the common difference 

 is H 2 . Such series have been called isologous%. The following pairs of 

 compounds are isologous with each other: — tritylic alcohol, C 3 H 8 O, and 

 allylic alcohol, C 3 H 6 O ; propionic acid, C 3 H 6 O 2 , and acrylic acid, C 3 IV O 2 ; 

 valeric acid, C 5 H 10 O 2 , and angelic acid, C 3 H 8 O 2 ; caproic acid, C° H 12 O 2 , 

 and sorbic acid, C 6 H 8 O 2 (difference =2H 2 ); sebacic acid, C 10 H 18 O*. and 

 camphoric acid, C 10 H 16 O*. 



If we confine ourselves to the comparison of bodies of the same chemical 

 function, we can seldom find more than two or three which belong to the 

 same isologous series ; but if we compare together entire groups, we per- 

 ceive the existence of considerable numbers of groups isologous with each 

 other. It would be easy to render this evident by constructing a Table in 

 which the various groups corresponding to differences in the values of n and 

 x should be arranged so as to show at a glance their mutual relations of 

 homology and isology ; the groups corresponding to variations in the value 

 of n, while that of x remains constant (homologous groups), being arranged 

 in the same vertical column ; and those which correspond to the same value 

 of n, but to various values of x (isologous groups), in the same horizontal 

 line, or vice versa. 



Gerhardt, in his « Traite de Chimie Organique,' arranges all the chemical 

 groups, which he includes in his scheme of classification, about two primary 

 homologous series, — the acetic series and the benzoic series. In Kekule's 

 ' Lehrbuch der organischen Chemie,' an arrangement is adopted which is 

 intermediate between that of Gerhardt and the classification in homologous 

 and isologous series described above. Kekule takes as the basis of his 

 arrangement, the three primary series of homologous hydrocarbons, of which 

 the first terms are — 



C 4 H 8 C 5 H 10 



Butylene. Amylene. 



C 7 H 8 C 8 H 10 



Toluole. Xylole. 



C !0 H 8 



Naphtaline. 



It will be seen that there is a common difference of C 4 H 2 between the first 

 terms of each of these series, and that, between the terms of the three series 



* Ann. Chim. Phys. [3] xlvii. 297; Jahresber. viii. 678. 



t Schiel, Ann. Chern. Pharm. xliii. 107 (1842), first pointed out the existence of sub- 

 stances possessing similar properties and differing in composition by C H 2 , or a multiple 

 thereof, This relation was afterwards shown by Gerhardt, Precis de Chimie Organique, 

 (1844-45), to be of very frequent occurrence, and was distinguished by him by the name 

 1 Homology.' 



% The word 'isology' is used by Gerhardt (Traite, i. 127) in a somewhat less restricted 

 sense. Gerhardt calls substances isologous which have the same chemical function, but 

 which do not present the relation of homology ; e. g. acetic acid, C 2 H 4 O 2 , and benzoic 

 acid, C? H« O 2 . 



