264 Chemistry. [Lecture 33. 



rytes. Acetat of lead is the next powerful. 

 Bergman observes, that paper stained with lit- 

 mus is reddened when dipped into water contain- 

 ing T _L._th part of sulphuric acid. 



Sulphur is, however, as I observed, sometimes 

 contained in a volatile or gaseous state. In this 

 it will be sufficiently known by its pungent and 

 suffocating smell, which is like that of burning 

 sulphur. Sulphuretted hydrogen gas is the most 

 common form which sulphur assumes in mineral 

 waters. It reddens the tincture of litmus ; but 

 the most certain tests are, that it blackens paper 

 dipped in a solution of lead, and precipitates the 

 nitrat of silver black or brown. 



Any acid in a fixed state, and in any quan- 

 tity, if mixed with a mineral water, may be dis- 

 covered by its sour taste, by the infusion of vio- 

 lets, or by an infusion of litmus. This is the 

 most delicate test ; but it has so much sensibility 

 to acids, that it may perhaps be affected by them 

 in a state of combination. This and the syrup 

 of violets may either be used in a liquid form, 

 or by being rubbed on paper and dried. Pieces 

 of paper are indeed the best, as we can compare 

 what is dipped in the water with what is left dry. 

 Alum, however, changes the colour of the in- 

 fusion of litmus as well as an acid. The infusion 

 is a deep purple ; but if alum is added, it im- 

 mediately turns red. But alum does not change 

 the infusion of violets ; and as no substance re- 

 tains its acid more loosely than alum, we may 



