Salts,. 1 45 



stantly as that of the other two ; for, if applied 

 to the skin of an animal, the heat soon causes 

 it to fly off in vapour ; but if confined by an ad- 

 hesive plaster, it has the same acrid effects as 

 soda and potass. It has a considerable attrac- 

 tion for water, and sometimes crystallizes ; but 

 the crystals cannot be easily separated from the 

 water, the alkali being more volatile than the 

 water, and being converted into vapour before 

 the water. We must therefore have recourse 

 to the retort, and distil the solution when we 

 want to separate the alkali from the water. In 

 the first operation we can perhaps obtain one- 

 half of the alkali ; and if we repeat it, we shall 

 obtain it complete and solid. 



Ammonia is sometimes produced by a natural 

 process, as is easily discoverable in large stables 

 which are kept close, where it affects even the 

 eyes on entering. There is indeed always a con- 

 siderable quantity in the air where there are a 

 number of animals together. This is proved 

 by exposing to the air any salt that has a strong 

 p.ttraction for it. It is only met with in a com- 

 pound state in nitrate or muriate of ammonia, 

 the latter of which is the common sal ammoniac, 

 and the former is sometimes found in the neigh- 

 bourhood of volcanoes, or coal mines that have 

 burnt for a long tune. For use, the volatile al- 

 kali is chiefly produced from animal substances, 

 particularly bones, or from soot, and putrified 

 vegetables ; but it is most convenient to prepare it 



VOL. II. H 



