96 Gerhardi’s Organic Chemistry. 
potato-oil and ethal, are examples; their composition -is respec- 
tively CH,O,, C,H ,O, C,H,,O and C i 
If the single equivalent of oxygen which each of them con- 
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 homologues ; 
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 t 
fatty bodies ; but their resemblances are only anal®gies, 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 0, and the other O,, or one N and 
the other N,, cannot be homologues, while bodies containing 
C,, or ©; and H,, or H,,; 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,, RO,, &c. Those containing nitrogen are 
and hydrogen must also be identical. Formic acid CH, 02; 2° 
tic acid C, H, O,, valerianic acid C, H,,O,, and ethalie acid 
C,,H,, O, are designated by the general formula RO,, and in 
7 ern etey 
