

96 GerhardVs Organic Chemistry. 



potato-oil and ethal, are examples ; their composition is respec- 

 tively CH<0 2 , C 2 H 6 0, C 5 H , 2 and C, ? H 34 * 



If the single equivalent of oxygen which each of them con- 

 tains, were united with two equivalents of the hydrogen to form 

 water, the carbon and hydrogen in the residue of each would be 

 in the proportion of 1 to 2. By oxydizing agents the alcohols 

 lose two equivalents of hydrogen and gain one of oxygen, giviti 

 rise to the formic, acetic, valerianic and ethalic acids, in each of 

 which the carbon and hydrogen are in the proportion of one to 

 two ; and in all the products of the transformation of these bodies, 

 the proportions of these elements still bear a similar relation to 

 each other. Hence if we know the composition of any deriva- 

 tive of spirit of wine, we can at once foresee that of a similar 

 product derived from any other body of the group. 



Substances like these having a likeness in characters depending 

 upon a similarity of constitution are denominated homologies ; 

 and are to be carefully distinguished from those which resemble 

 each other merely in physical characters, and which are called 

 analogues. For example, wood-spirit resembles acetone in being 

 inflammable, odorous, very volatile, and soluble in water, while 

 ethal is allied to stearine in being solid at ordinary temperatures, 

 insoluble in water and having other properties common to the 

 fatty bodies ; but their resemblances are only analogies, and when 

 we examine wood-spirit and ethal in relation to their constitution 

 and the products of their decomposition, we find that they are 

 closely related to each other and are homologues. 



In homologous bodies, the combustible elements, carbon and 

 hydrogen vary exceedingly in their proportions, while the oxygen 

 and azote are always atomically the same. Two bodies there- 

 fore which contain the one 2 and the other 3 , or one N and 

 the other N 2 , cannot be homologues, while bodies containing 

 C 9 , or C 5 and H , ■■ or H 3 4 , may very well be so, as in the alco- 

 hols already mentioned. M. Gerhardt has adopted some general 

 formulas to express these relations ; R, representing the carburets 

 of hydrogen ; RO, those bodies which like alcohol, contain one 

 equivalent of oxygen ; while other oxygenized compounds are 



designated as RO ? , R0 3 , & c . Those containing nitrogen are 

 represented in a similar manner, thus RN, RN 3 . 



In order that two or more bodies may be homologues, it is not 

 sufficient that they can be represented by the same general for- 

 mula; the equivalent ratio between the proportions of carbon 

 and hydrogen must also be identical. Formic acid CH 2 , ace- 

 tic acid C 2 H 4 2 , valerianic acid C.H, O , and ethalic acid 



°j^ H 32 2 are designated by the general formula R0 2 , and in 



* In these formulas it will he observed that our author divides the equivalent " f 

 Hydrogen, representing water by H 2 O. The equivalent of most of organic com- 

 pmmds .s taken at one-half the number usually adopted, for reasons which wi" 

 Pe explained farther on. r 



