1820.] Dr. Thomson on Arsenic. 83 



The specific gravity of these crystals is 1"759, that of water 

 being reckoned I'OOO. Now if the salt be a compound of 1 atom 

 arsenic acid, 1 atom soda, and 20 atoms water, as we shall im- 

 mediately find reason to conclude its constituents to be, the mean 

 specific gravity of such a compound (supposing no condensation) 

 would be 1*07 8. Hence it appears that the 22 volumes of which 

 it consists are condensed into about two-thirds of their original 



One hundred grains of water at the temperature of 45° dissolve 

 10*132 gr. of this salt after being deprived of its water of crys- 

 tallization. Now this is equivalent to 22-268 gr. of the crystal- 

 lized salt. The specific gravity of this liquid (containing very 

 nearly -j-'t^^ of its weight of the dry salt) at the temperature of 

 60° is 1-0903. Now the specific gravity of such a solution, 

 supposing no condensation to have taken place, would have 

 been 1*0698. Thus there is a condensation which scarcely 

 exceeds two per cent. The salt is much more soluble in water 

 of the temperature of 60°, and when the temperature amounts to 

 120°, the liquid dissolves more than its own weight of the crys- 

 tallized salt. 



This salt does not dissolve in alcohol; but when a crystal of it 

 is suspended in that liquid, its surface is deprived of its water of 

 crystallization, which renders it opaque and white. 



When heated, it speedily undergoes the watery fusion, the 

 water of crystallization being more than sufficient to keep it in 

 solution at a boiling temperature. When kept for some time in 

 a temperature of between 500° and 600°, it loses the whole of its 

 water of crystallization, and is converted into a white powder. 

 When this powder is exposed to a red heat, it undergoes the 

 igneous fusion, and becomes liquid and transparent like water. 

 By this treatment it sustains an additional loss of weight; but 

 this loss is partly at the expense of the acid of the salt, which 

 seems to undergo a partial decomposition ; for the salt after this 

 treatment is no longer completely soluble in watei-. 



The greater number of the arseniates are insoluble in water. 

 Hence the arseniate of soda occasions a precipitate when 

 dropped into most of the earihy or metallic salts. The following 

 table will put the reader in possession of the colour, solubility in 

 nitric acid, &c. of the most remarkable of these precipitates. 



Effect produced by dropping a saturated Solution of Arseniate of 

 Soda at 4i)° into various saline Solutions. 



1 . Muriate of Barytes. — Becomes slowly milky, and a white 

 precipitate falls. Redissolved by nitric acid. 



2. Muriate of Lime. — A white precipitate. Redissolved by 

 nitric acid. 



3. \ilrate of Strontian.—^A white precipitate. Redissolved 

 by nitric acid. 



f 2 



