158 HISTORY OF CHEMISTRY. [LECTURE IX. 



"hypothesis," to get rid of the division, which had subsisted 

 up to this time, into hydrogen acids and oxygen acids. This 

 hypothesis is a reversion to the ideas of Davy and of Dulong. 1 *' 1 

 A similar attempt had previously been made by Clark, 

 although much less elaborated. According to Griffin, 17 Clark 

 stated views of this kind in his lectures as early as 1826. As 

 he himself wrote to Mitscherlich in i836, 18 he finds grounds 

 for his opinion in the isomorphism of sulphate of soda and 

 permanganate of baryta. At that time, the formulae assigned 

 to these compounds were : 



NaOSO 3 and BaOMn 2 O 7 , 



according to which they contained an unequal number of 

 atoms. Clark proposes to double the atomic weight of 

 sodium (that is, to assume it to be four times as great as at 

 present) and to assign to it the number that Berzelius adopted 

 in iSiQ. 19 Since he further regards the acids as hydrogen 

 compounds, from which salts are produced by the replacement 

 of the hydrogen by metals, sulphuric acid, with him, is H 2 SO 4 

 and permanganic acid HMnO 4 ; and therefore sulphate of soda 

 is NaS 2 O 8 , and permanganate of baryta is BaMn 2 O 8 ; and by 

 this means he attains similarity in the number of atoms in both 

 compounds. 



Quite different grounds, which are of much superior value, 

 and are more numerous, lead Liebig to revive again the Davy- 

 Dulong hypothesis. Graham had shown that pyro- and meta- 

 phosphoric acids can exist in aqueous solution without at once 

 passing into ordinary (tribasic) phosphoric acid. Liebig con- 

 sequently inquires whether these three acids really differ from 

 one another by an atom of water in each case, and whether it 

 is the gain or loss of water which brings about the changes in 

 the basicity of phosphoric acid. He does not believe that 

 convincing grounds can be found for the adoption of this 

 hypothesis ; so that the contrary supposition, in accordance 



16 Compare p. 83. 17 Griffin, The Radical Theory in Chemistry, 



London (1858), 4 et seq. 18 Annalen. 27, 160. 19 Compare p. 94. 



