156 HISTORY OF CHEMISTRY. [LECTURE IX. 



Liebig has recourse to experiment in order to decide this 

 highly important question. He compares the behaviour of 

 phosphoric acid with that of sulphuric acid, a compound con- 

 cerning which he has no reason for reckoning it in this class. 

 In doing so he says : n - 



" If to acid sulphate of potash, we add another base which 

 is not isomeric with potash and which forms with sulphuric 

 acid a salt free from water of halhydration, 12 soda for example, 

 the acid salt separates into two neutral ones, Glauber's salt 

 and sulphate of potash, which crystallise apart from each 

 other. 



"If, on the other hand, a certain quantity of potash is 

 added to acid phosphate of soda, phosphate of soda and 

 potash is formed, wholly analogous in its composition to the 

 acid salt. It contains three atoms of base ; two of these are 

 soda and potash ; one of the two atoms of water previously 

 contained in it, is replaced by potash, the second atom remains 

 in the composition of the new salt. 



"This behaviour distinguishes phosphoric acid and arsenic 

 acid from the great majority of all other acids : their power of 

 forming salts of the same class with different bases, differing 

 from those which are called double salts, depends essentially 

 upon their property of combining with several atoms of base. 

 I regard this character as decisive respecting the constitution of 

 these, and of all acids which form compounds similar to those of 

 phosphoric acid" 



A criterion is thus found for separating phosphoric acid and 

 its analogues from the other acids, and Liebig employs it in 

 order to establish the fact that all the substances examined by 

 him belong to this class. The grounds upon which he also 

 decides to include tartaric acid in this group are very interest- 

 ing and important. This acid was at that time written C 4 H 4 O 6 , 

 so that its atom saturated only one atom of base. The exist- 



11 Annalen. 26, 144-145. 12 Liebig regards as water of halhydration, 

 that water in salts which can be separated and replaced by equivalents of 

 neutral salts. 



