38 Dr. W. Odliiig on the Doctrine of Equivalents. 



nicna of what was then considered chemical combinationj as 

 instanced more particularly in the case of neutralization. Thus, 

 a given proportion of potash united with an equivalent quantity 

 of oxalic acid to form a neutral salt, and with twice its equiva- 

 lent to form an acid salt, and so forth. But this idea of equi- 

 valency, which accords much the same signification to the words 

 atom and equivalent, has been greatly called in question. The 

 controversy was begun by Gerhardt and Laurent, who first 

 clearly established the diffei-ence between the atomic weight of a 

 body and its equivalent value. Subsequently Williamson and 

 I have published papers on the subject ; but it is only within 

 these recent days of polyatomic radicals, that the distinction has 

 begun to receive the general recognition which it merits, as a 

 fundamental doctrine in chemical philosophy. 



We know that no two substances in nature are absolutely 

 equivalent, but that those are most nearly equivalent w^iich 

 manifest the greatest resemblance as regards some special func- 

 tion taken as the test of comparison ; and that the prime indi- 

 cation of equivalency between any two or more substances con- 

 sists in their capability of being exchanged for one another. 

 Hence we look for typical illustrations of chemical equivalency, 

 not so much among dissimilar bodies which can neutralize one 

 another, as among similar bodies which can replace one another. 

 The idea of direct or indirect substitution forms the basis of the 

 modern doctrine of chemical equivalents. Thus we find in a 

 great number of compounds, that 1 part of hydrogen may be 

 replaced directly by 23 parts of sodium, or by 39 parts of potas- 

 sium, or by 32*5 parts of zinc, or by 55"7 parts of cadmium, &c.; 

 and as the resulting compounds manifest a marked mutual re- 

 semblance of constitution and properties, and a general resem- 

 blance to the hydrogenized body from which they wei-e obtained, 

 we say that the atoms of sodium, potassium, zinc, and cadmium 

 are equivalent to one another, and also equivalent to an atom 

 of hydrogen. Similarly, we find in a great number of compounds, 

 both organic and inorganic, that one or more atoms of hydrogen 

 may be replaced by an equal number of atoms of chlorine, or 

 bromine, or iodine ; and as the resulting compounds correspond 

 closely with one another, and present a marked similitude to the 

 original hydrogenized body, we say that the atoms of chlorine, 

 bromine, and iodine are equivalent to one another, and also 

 equivalent to an atom of hydrogen, whence it follows that the 

 atoms of chlorine, bromine, iodine, hydrogen, potassium, sodium, 

 zinc, cadmium, &c., arc mutually equivalent. We are acquainted 

 with numerous bodies of comparable constitution, containing 

 respectively an atom of chlorine or an atom of metal, thus* : — 

 * C=12. 0=16. S=32. 



