XVI. ] ON THE THEORY OF CHEMICAL CHANGES. 431 



and evidently offers no exception to the law of equivalent vol- 

 umes. 



The law of Laurent, that the number of atoms of hydrogen, 

 or of hydrogen, chlorine, nitrogen, metals, etc., in any formula 

 corresponding to four volumes of vapor, is always a sum divisi- 

 ble by two, clearly follows from the principles already laid 

 down, and from the fact that nitrogen and the metals are 

 subject to the same conditions as hydrogen and chlorine ; the 

 atoms have the value which has been assigned to H and to Cl 

 in the formulas given above. The same rule of divisibility, as 

 Laurent has already shown, necessarily holds in regard to the 

 number of atoms of carbon, as well as to the oxygen and sul- 

 phur, if we take for their equivalent weights the numbers 6, 8, 

 and 16 respectively.* 



It is to be remarked that while the coefficients of H, Cl, or 

 !KT, in formulas where these are associated, may be odd num- 

 bers, those of 0, S, and C are always even. This seems a 

 conclusive reason for doubling the equivalents of the latter, or 

 dividing those of hydrogen, chlorine, the metals, etc., according 

 as four or two volumes are taken for the equivalent. [/See p. 176.] 



I have elsewhere pointed out that carbon and oxygen sustain 

 such relations that C 2 H 2 may be compared with 2 H 2 and with 

 2 M 2 , and, by the substitution of nitrogen for hydrogen, with 

 C 2 HN, prussic acid, and 2 N 2 , nitrous oxide (the so-called 

 compounds of nitrous oxide with bases are probably 2 MN, 

 corresponding to the cyanides, C 2 MN) ; while the peroxide of 

 hydrogen, 4 H 2 , corresponds to 4 N 2 , nitric oxide, and to C 4 N 2 , 

 cyanogen. This relation has important bearings on the history 

 of the cyanic series, and the nitric derivatives of the hydro- 

 carbons, t 



The formulas of such related species as Gerhardt has desig- 

 nated chemical homologues differ from each other bynC 2 H 2 ; 



* Laurent, Kecherches sur les combinaisons azotees, Ann. de Chimie et de 

 Physique, November,. 1846; and American Journal of Science for September, 

 1848, p. 174. 



t See page 502 of my Introduction to Organic Chemistry, appended to 

 Silliman's First Principles of Chemistry, Phila., 1852 ; and the above Journal 

 for January, 1853, p. 151. 



