Dr. W. Oclling on the Duclrine uf Equivalents. il 



In the above formulae, despite the different numbers of the 

 atoms, the same equivalent or exponential value is maintained 

 throughout. Equivalency is quite independent of an equality 

 of atoms, but relates solely to functions and relations. When 

 the same body has several functions, it is likewise possessed of 

 several equivalents. The majority of chemists have heretofore 

 confounded the equivalent value of a body with its atomic 

 Aveight, its equivalent with its atom, which is, I conceive, un- 

 warrantable. Thus K represents an atom or proportion of 

 potassium, and KH SO* an atom or proportion of acid svdphate 

 of potash ; but there is no eciuivalency between the two bodies. 

 On the other hand, K''^ represents three atoms of potassium, and 

 Bi'" one atom of bismuth, but the two formulfe correspond to the 

 same equivalent or representative value. In order to have equi- 

 valency, we must have some analogy in constitution or properties, 

 to a definite standard of comparison. 



But our idea of equivalency is not by any means confined to 

 elementary bodies. It is applicable to all the varieties of com- 

 pounds, real or hypothetical, with which chemists are acquainted, 

 and more particularly to those groupings which are denomi- 

 nated compound radicals. Whether or not we admit the inde- 

 pendent existence of preformed compound radicals, it is indis- 

 putable that certain groupings of atoms are capable of being 

 transferred from one combination to another in exchange for 

 an atom of hydrogen, or chlorine, or metal ; or in other words, 

 are capable of replacing hydrogen, chlorine, or metal, by equi- 

 valent substitution. Thus we can transfer the molecular gi'oup- 

 ings that we denominate cyanogen, CN ; amidogen, NH"^ ; 

 peroxide of hydrogen, HO ; benzoyle, C*" H^ ; othyle, C^ H^ ; 

 cthyle, C^ H'^; amyle, C^ H", &c., from one compound to another, 

 in exchange for one another, or for hydrogen, or its representa- 

 tives, with a facility by no means inferior to that with which 

 we effect actual elementary substitutions, For a long time past 

 these groupings have been looked upon as the representatives 

 or equivalents of one atom of hydrogen, but it is only of late 

 years that there has been recognized among compound radicals 

 that multifold equivalency, which forms the basis of the so- 

 called polyatomic radical theory, and which has recently been so 

 much employed by Ilofmann and Cahours, Wurtz, Berthelot, 

 and others, as the exponent of many brilliant researches. 



The idea of mult-equivalent or polyatomic radicals was, I 

 believe, first announced by A^'illiamson in a paper " On the Con- 

 stitution of Salts," published in the year 1851. Some time 

 afterwards he found a disciple in myself, who, in 1854 and 

 1855, extended his original notion, ilhistratcd it by a variety 

 of formula*; and, moreover, rendered it more i)recise by «pj>ly- 



