PICRIC ACID. 187 



Carbon 12 atoms .... 31*44 



Hydrogen 2 .... 0'87 



Nitrogen 3 .... 1834 



Oxygen 13 .... 45'42 



Water 1 .... 3'93 



100-00 



The atomic weight of the hypothetical anhydrous acid= 2750*0; 

 and its saturating capacity = 3*636. Chemists are not agreed 

 regarding the rational formula of this body; they unite in regarding 

 it as a conjugated nitric acid, but there is much difference of opi- 

 nion regarding the nature of the adjunct. Berzelius writes this 

 acid as= (C 12 H 2 NO 3 .NO 5 ) +NO 5 .HO, but there is little to support 

 the view of a salt-like adjunct such as is here assumed. We know, 

 for instance, that the group of atoms NO 4 is substituted in aniline 

 and certain other bodies for an equivalent of hydrogen, and it is 

 now pretty generally assumed that such substitutions of more nega- 

 tive matters in the place of hydrogen for the most part only extend 

 to the hydrogen contained in the adjunct ; if, therefore, we assign 

 to picric acid only a hypothetical formula, it will at all events not 

 be an irrational one, if we consider that in the adjunct C 12 H 4 , 

 2 atoms of hydrogen are replaced by 2 atoms of NO 4 , arid write the 

 acid as = C 12 (H 2 .2NO 4 ).NO 5 .HO. Laurent regards picric acid, not 

 as a conjugated acid, but as carbolic acid (C 12 H 5 O) in which 3 

 atoms of hydrogen are replaced by 3 atoms of NO 4 , and hence he 

 writes it as = C 12 (H 2 .3NO 4 )O.HO. 



Combinations. The picrates are crystallisable, yellow, and for 

 the most part soluble in water ; when rapidly heated they decrepi- 

 tate with much violence. 



Pier ate of potash is one of the most insoluble salts of this acid; 

 it crystallises in long, glistening, yellow, iridescent prisms, and 

 dissolves in 260 parts of cold, and 14 parts of hot water. With 

 alkaline earths and metallic oxides this acid has a tendency to form 

 basic and very insoluble salts. 



Preparation. This acid is formed by the action of concentrated 

 nitric acid on many vegetable and animal substances. Thus, for 

 instance, in heating salicin with nitric acid, we obtain crystals of 

 pure picric acid. It is likewise produced in large quantity on 

 decomposing silk with nitric acid ; it is, however, most commonly 

 obtained by boiling indigo with nitric acid. 



