98 



Gerhard? $ Organic Chemistry. 



The salicylic acid C 7 H 6 3 , and the anisic 



C 8 H 3 3 , are 



monobasic and contain three equivalents of oxygen ; in the first, 

 the deficiency of hydrogen is 14-6 = 8, and the second =16 — 

 8 = 8. These acids may then be represented by the formula R" 8 3 . 



This proportion between the elements of a compound does 

 not, however, necessarily imply a homology ; there are some ex- 

 ceptions which depend in some way upon the peculiar grouping 

 of the elements. Thus ordinary ether C 4 H , O, is represented 

 by the same general formula as alcohol R+ 2 O, but the chemical 

 characters of the two are entirely different and do not allow us to 

 consider them homologues. It is then necessary to add as a con- 

 dition of homology, a similarity of chemical characters, dependent 

 upon a like arrangement of the molecules. Vol. i, pp. 29-35. 



This notation expresses in a beautiful and simple manner, the 



pounds. 



It is the peculiarity of this system that it is based upon the natu- 

 ral affinities of bodies and not upon analogies ; this is the only 

 arrangement which will always be correct, because it is founded 

 in the constitution of the substances themselves. 



The important relations which the combustible elements sus- 

 tain, appear "to permit us to class homologous bodies according 

 to their carbon/' and M. Gerhardt has accordingly constructed 

 upon this basis a classification in which all organic substances 

 are arranged in a tabular form. Those containing the same 

 atomical proportion of carbon constitute a family which is de- 

 signated by the number of equivalents of that substance. Each 

 family is divided into the carburets of hydrogen and those con- 

 taining oxygen and nitrogen, so that we have R, R0 2 , RN, &c. 

 These divisions are found on the left of the table, while at the 

 top are marked at the head of their respective columns, the pro- 

 portions of hydrogen. This will be better understood by a view 

 of a part of the 1st and 2d families. 



Family. | Gen- formula 



R+2 



r 



R 



RO 



2. <! 



RO 



C 2 H 6 , 



acetene. 



C 2 H 6 0, 



(a) alcohol. 



(b) metl. ether 



2 



RO 



R 



1. 



RO 



RO 



CH 4 , 



marsh gas. 

 CH 4 0, 



wood-spirit 



C 2 H 4 , 



olefiantgas. 



C 2 H 4 0, 



aldehyde. 



C 2 H 4 2 , 



acetic acid. 



( C 2 H 2 



( oxalic acid. 



CH 2 2 , 



formic ac 



id. 



CO, oxyd 

 of carbon. 

 C0 2 , car- 

 bonic acid ga* 



