122 ESSAY VI. 



into the neck of the retort; then the residuum was boiled 

 in water, upon which the acid dissolved, but the silex 

 remained at the bottom unchanged. 



SECTION XX. UPON TERRA PONDEROSA. 



The earth in the ponderous spar is not lime, as is 

 generally believed, but a peculiar kind of earth. It is 

 the same with that of which I related some experiments 

 in my Dissertation on Manganese. Mr. I. G. Gahn after- 

 wards informed me that he found that this earth was the 

 basis of the ponderous spar. Incited by this, I procured a 

 larger quantity of it, in order to make more experiments. 

 I shall here only mention the effects of acid of arsenic. 

 (a) Terra ponderosa dissolves readily in the acid of arsenic ; 

 but as soon as it has obtained its point of saturation it 

 again precipitates, combined with the arsenical acid. (&) 

 Neither is its solution in nitrous, muriatic, acetous acid 

 precipitated by the acid of arsenic ; but the arsenical 

 neutral salts (Sees. vi. (a), vii. (a), vm. (a)) precipitate these 

 solutions. (c) In the crucible this precipitate shows the 

 same phenomena as the precipitate of lime ; as also when 

 treated with charcoal powder, (d) The solution of ponderous 

 earth in acid of arsenic is again precipitated by the acid of 

 vitriol, in the form of a salt insoluble in water, viz. re- 

 generated ponderous spar. 



SECTION XXI. THE EFFECTS OF THE' ACID OF ARSENIC 

 UPON METALS. 



As the acid of arsenic cannot be kept long in the 

 crucible without being converted into arsenic, in which 

 state it sublimes (Sec. v. (&)), and as in a retort it continues 



