128 THE SECRETIONS: 



faces. To ascertain more precisely the nature of the acid 

 combined with the oxide of zinc in this salt, the crystals were 

 separated as carefully as possible from the mother-ley, pressed 

 between blotting-paper, dissolved in a large quantity of boiling 

 water, in which they were but sparingly soluble, and allowed to 

 crystallize by cooling. The mother-ley afforded more crystals 

 on further evaporation. They were again separated from ad- 

 hering liquid by pressure. 



" The zinc salt thus obtained had a faint greenish-yellow tint, 

 and was therefore probably not quite pure, although its solution 

 was perfectly colourless. The acid was isolated from this salt 

 by means of sulphuretted hydrogen ; after separation of the 

 sulphuret the solution was entirely free from zinc. The liquid, 

 which had a strong acid reaction, was freed by boiling from the 

 excess of sulphuretted hydrogen, and evaporated on the water- 

 bath. When it had become sufficiently concentrated, the acid 

 separated in prismatic crystals, which appeared to form quadri- 

 lateral rectangular columns and tables. It is easily soluble in 

 water, and separates in crystals on evaporation; the solution has 

 a strong acid taste, and reddens litmus-paper. It likewise dis- 

 solves in alcohol, but not quite so easily as in water ; ether 

 dissolves scarcely a trace of it. Heated on platinum foil it melts, 

 becomes brown, and leaves behind a coal, which is difficult of 

 combustion, but which disappears entirely by stronger heat. 



" From the mode of preparation it is evident that the acid 

 forms with oxide of zinc a very sparingly-soluble salt, which 

 separates in microscopic crystals. When the acid is supersatu- 

 rated with ammonia, and the solution evaporated on the water- 

 bath, so much ammonia escapes that it again becomes acid; if 

 it be evaporated to dryness, so that all the ammonia that could 

 escape at this temperature is expelled, and caustic potash be 

 added to the mass, a considerable quantity of ammonia is 

 given off; therefore it appears that this acid, like many organic 

 acids, forms acid salts. The ammonia-salt obtained in this 

 manner is somewhat more difficult of solution in water than the 

 acid itself. When the acid is accurately neutralized with pot- 

 ash, it forms an easily- soluble salt, the solution of which affords 

 no precipitate with sulphate of copper. The oxide of copper is 

 not thrown down from this mixture by an excess of potash, but 

 the colour of the solution becomes somewhat darker. Acetate 



