266 ESSAY XXIV. 



a very small quantity of caustic alkali ; for the greater part 

 of the Glauber's salt shoots again into crystals. If they be 

 again boiled, some alkali will indeed be obtained, and so on 

 continually ; but, with what immense trouble is this mode 

 of proceeding attended ? 



Concerning what Mr. Hermbstaedt has said (Part 9, p. 

 66) on the subject of lapis infernalis, it may be remarked 

 that the dark colour of this caustic arises from copper ; for 

 even the finest silver-leaf contains copper, which is dissolved 

 at the same time ; but in the melting heat the copper loses 

 its solvent sooner than the silver, and being then in the 

 state of a calx, it acquires its natural dark colour ; and for 

 this reason it is that lapis infernalis is commonly black. If 

 it be dissolved in water, a black powder will remain behind, 

 which is nothing but calx of copper. 



LETTER FROM THE SAME TO THE SAME. 1 



I believe that the volatile sweet matter contained in 

 various oily and fat substances (p. 262) may be totally 

 destroyed by repeated distillations. I have at least obtained, 

 after every rectification, a product resembling spirit of tartar, 

 and the sweet substance became every time more acrid and 

 bitter. The strong heat which is necessary in order to drive 

 over, is the cause of this destruction. 



Mr. Kirwan's experiments, which are adduced to prove 

 that the aerial acid consists of phlogiston and pure air, seem 

 liable to some objections, as long as iron-filings are used for 

 the revivification of red precipitate, and a burning-glass for 

 the calcination of iron in pure air ; for iron commonly 

 contains some aerial acid, as it is more or less mixed with 

 plumbago, and as this mineral consists of phlogiston and 

 1 Chemische Annalen, St. x. p. 328. 



