112 ESSAY VI. 



found to be covered in the inside with a white glazing ; a 

 salt still remained, which, after being dissolved in water 

 and evaporated, appeared to be the same salt, with excess 

 of acid, as before, (d) This neutral salt, as it is called, was 

 mixed with one-eighth of charcoal powder, and distilled in 

 a glass retort. When it began to grow red hot, it set to 

 boil violently, without taking fire, and a very fine regulus 

 of arsenic was sublimed. The black residuum, when dissolved 

 in water, proved to be of an alkaline nature, and showed no 

 vestige of the acid of arsenic. Mr. Macquer thinks that 

 his neutral salt is an alkali mixed with arsenic in some 

 peculiar manner; but it now appears that it is only one 

 of the constituent parts of arsenic with which the alkali is 

 combined. And this is likewise . the reason why acids do 

 not precipitate any arsenic from this salt. 



SECTION VII. MINEKAL ALKALI. 



(a) If mineral alkali be saturated with the acid of 

 arsenic, crystals are obtained, the form of which is entirely 

 the same with that of those just mentioned ; but they do 

 not change lacmus, and they make syrup of violets green. 

 (6) If acid of arsenic be added, no crystallisation ensues, 

 but the mass, after being evaporated to dryness, grows 

 moist again in the air. 



SECTION VIII. VOLATILE ALKALI. 



(a) Volatile alkali, saturated with the acid of arsenic, 

 affords, upon evaporation in the open air, an arsenical sal 

 ammoniac, which in its form is very like the foregoing 

 salts. This salt does not change lacmus, but turns the 

 syrup of violets green ; it loses its transparency when 



