BENZOIC ACID. 851 



centig.) the density of its vapour is 4270 by experiment, and 

 4260 by calculation. Heated in open air it allows a white 

 vapour to diffuse, which greatly irritates the fauces and pro- 

 vokes coughing. It is very inflammable, and burns with a white 

 smoky flame without leaving any residue. 



Benzoates. Benzoic acid is readily dissolved by alkalies and 

 alkaline carbonates, and also by phosphate of soda. Several 

 insoluble benzoates are dissolved, according to Lecanu and 

 Serbat, by acetates of potash and soda and nitrate of soda, 

 while they are insoluble, in nitrate and sulphate of potash, and 

 sulphate of soda. Ammonia forms a neutral and acid benzoate ; 

 the last presents itself in large regular crystals. The salts of 

 potash, soda, lithia, and magnesia are very soluble, and crystallize 

 with difficulty. The salt of lime is soluble in 20 parts of cold 

 water, and in a greater proportion in boiling water ; it crys- 

 tallizes in flexible needles, or brilliant prisms, which contain I 

 atom of water of crystallization. Benzoate of alumina is 

 obtained as a crystalline precipitate. Benzoates of manganese 

 and protoxide of iron are pretty soluble. Neutral benzoate of 

 peroxide of iron, Fe 2 O 3 -f3BzO, is soluble in water and alcohol 

 and crystallizable. But peroxide of iron is thrown down in the 

 form an insoluble sub-benzoate, of a reddish white or buff colour, 

 when a soluble benzoate is added to a solution of peroxide of 

 iron, previously neutralised without precipitating any peroxide of 

 iron by means of ammonia. To prevent the decomposition of 

 this precipitate and the formation of a soluble benzoate of iron, 

 it should be washed by a solution of sal ammoniac. Benzoate 

 of ammonia is often employed to separate peroxide of iron from 

 oxides of manganese, nickel, and zinc, but when the solution 

 contains at the same time alumina, yttria, zirconia, or glucina, 

 the process does not answer, as the benzoates of these oxides 

 are equally insoluble with that of peroxide of iron. Benzoate 

 of lead is nearly insoluble in water, but soluble in acetic acid, 

 from which it may be obtained crystallized in plates, which 

 contain 2 atoms of water, one of which it abandons when dried 

 at 212. Solutions of an alkaline benzoate and trisacetate of 

 lead, give a white anhydrous and insoluble precipitate, which is 

 basic benzoate of lead consisting of 2 atoms of benzoic acid 

 united with 3 atoms of oxide of lead. Benzoate of silver is ob- 

 tained in colourless needles, flattened and brilliant, on treating 



2 K K 2 



