ARSENIC ACID. 62 ( J 



by heating powdered arsenious acid in a bason, with an equal 

 quantity of water, and adding to the mixture at the boiling point 

 nitric acid in small quantities, so long as ruddy fumes escape. 

 An addition of hydrochloric acid to the water is generally made, 

 to increase the solubility of the arsenious acid, but it is not ab- 

 solutely necessary. The solution of arsenic acid is then eva- 

 porated to dryness, to expel the remaining nitric and hydro- 

 chloric acids, but the dry mass is not heated above the melt- 

 ing point of lead, otherwise oxygen gas is emitted and arsenious 

 acid reproduced. Arsenic acid, thus obtained, is milk-white, 

 and contains no water. Exposed to air, it slowly deliquesces, 

 and runs into a liquid. But notwithstanding this, when 

 strongly dried, it does not dissolve completely in water at 

 once, and a portion of it appears to be insoluble ; but the 

 whole is dissolved by continued digestion. Arsenic acid, in 

 absorbing moisture from the air, sometimes forms hydrated 

 crystals, which are highly deliquescent ; but this acid is easily 

 made anhydrous, and does not retain basic water with force, 

 like phosphoric acid. Its solution has a sour taste, and red- 

 dens vegetable blues. Arsenic acid, indeed, is a strong acid, 

 and expels, with the aid of heat, all the volatile acids from 

 their combinations. Arsenic acid undergoes fusion at a red 

 heat, and is completely dissipated in arsenious acid and oxy- 

 gen at a higher temperature. 



When an equivalent of arsenic acid is ignited with an excess 

 of carbonate of soda, three equivalents of carbonic acid are 

 expelled, and a tribasic arseniate of soda formed, which crystal- 

 lizes when dissolved in water, with 24 equivalents of water, 

 forming the salt 3NaO, AsO 5 + 24HO, isomorphous with the 

 subphosphate of soda. The same salt is obtained by treating 

 arsenic acid in solution, with an excess of caustic soda. When 

 carbonate of soda is added to a hot solution of arsenic acid, so 

 long as there is effervescence, a salt is obtained by evaporation, 

 corresponding with the common phosphate of soda, containing 

 2 eq. of soda and 1 eq. of water as bases. This salt affects the 

 same two multiples, in its water of crystallization, as phosphate 

 of soda, namely 24HO and 14HO, but most frequently as- 

 sumes the smaller proportion, forming the salt 2NaO, HO, 

 AsO 5 -fl4HO. This arseniate is more soluble than the phos- 

 phate, and slightly deliquescent in damp air. When to the last 

 salt a quantity of arsenic acid is added, equal to what it already 



