420 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



Avogadro's hypothesis, which had been abandoned even by 

 Dumas and were only established finally when Deville's 

 experiments on dissociation (Chapter XX) had cleared the 

 ground for Cannizzaro's epoch-making exposition. 



Gerhardt (1856) distinguishes the hydrogen and chlorine 

 radicals from hydrogen and chlorine gas. It is typical of 

 the difficulties that had been introduced into chemistry by 

 Berzelius's dualistic theories that Gerhardt appeared to be 

 breaking new ground when he wrote the formula of hydrogen 

 gas as H 2 and the formula of chlorine as C1 2 . Berzelius 

 would not recognise that hydrogen could combine with 

 hydrogen or chlorine with chlorine, since there could 

 be no electrical polarity to produce an attraction between 

 similar atoms. He therefore discarded Avogadro's hypo- 

 thesis as applied to elements, even when he was prepared 

 to apply it to compounds. Dumas, on the other hand, 

 fell into error when he attempted to apply Avogadro's 

 hypothesis to the elements on the assumption that all 

 elementary molecules were diatomic, whence Hg=ioo, 

 P = 62 instead of Hg = 200, P = 3i. Gerhardt, however, saw 

 that a distinction must be drawn between the hydrogen 

 radical and hydrogen gas, between the chlorine radical and 

 chlorine gas. Thus he writes : 



" In opposition to most chemists, / regard the expression 

 radical in the sense of a relatiotiship, and not in that of a body 

 that may be or has been isolated, I distinguish, therefore, 

 the hydrogen radical from hydrogen gas, the chlorine radical 

 from free chlorine ; or better, if free hydrogen or chlorine 

 is to be represented by rational formulas, a study of their 

 reactions shows that hydrogen gas must be represented by 

 the two radicals HH, and chlorine gas by the two 

 radicals C1C1. In the usual nomenclature, hydrogen gas 

 would then be the hydride of hydrogen, and chlorine gas 

 would be the chloride of chlorine ; that is to say that 

 chlorine gas and hydrogen gas may be formed by, or may 

 give rise to, double decompositions precisely similar to those 

 which have caused oil of bitter almonds to be described as 



