l6o HISTORY OF CHEMISTRY. [LECTURE IX. 



Liebig admits that it is difficult to understand how potash 

 is reduced by means of sulphuric acid, an assumption which 

 must be made in case sulphate of potash is to be regarded as 

 a potassium compound ; but he instances a case in which a 

 hypothesis of the kind is indispensable to the explanation of 

 the facts. The decomposition of thiocyanate of silver by 

 means of sulphuretted hydrogen, with the formation of sulphide 

 of silver and free acid, would be contrary to all views regarding 

 affinity if the salt corresponded to the formula AgS + Cy 2 S ; 

 whereas the reaction becomes a normal one, assuming the 

 formula to be Ag. Cy 2 S 2 . Unsatisfactory as it is at first sight, 

 the hypothesis of the reduction of the oxides by means of acids, 

 further gives an explanation of the behaviour of many acids 

 which exhibit a higher capacity for saturation towards silver 

 oxide than they do towards soda, although the latter is endowed 

 with more strongly basic properties. 



Finally, Liebig points out that by the assumption of the 

 hypothesis of Dulong, the grouping of the hydrogen acids and 

 of the oxygen acids into one class, which is almost enforced 

 by the similarities in their reactions, is attained. Thus lime 

 always gives up the same quantity of water no matter whether 

 it is neutralised with sulphuric acid or with hydrochloric acid. 

 The mode of explanation then adopted, in accordance with 

 which the water, in the one case, was present in the acid ready- 

 formed, and in the other case was produced during the action, 

 according to Liebig takes no account of the analogy of the two 

 cases. He tries to pull down the barrier, and his words are 

 sufficiently significant to deserve a place here. 22 



" We employ, therefore, two modes of explanation for one 

 and the same phenomenon ; we are forced to ascribe to water 

 the most varied properties ; we have basic water, water of hal- 

 hydration, water of crystallisation ; we observe its entrance into 

 compounds in which it ceases to assume any one of these three 

 forms ; and all this is for no other reason than that we have 

 drawn a distinction between haloid salts and oxygen salts, 



22 Annalen. 26, 179. 



