^ On a native Arsmiate of Lead, 



nitric acid : as no arsenic acid bad united with the precipi- 

 tant. The solution of I he nitrate of ammonia was evapo- 

 rated to dryness, and exposed to a red heat in a platina 

 crucible ; but nothing was left, except a shght trace of 

 oxide of lead. Wc may infer from hence, the absence of 

 both the fixed alkaHes. 



4. I found in one specimen only of this fos&il any no- 

 table diti'erence in the relative proportions of the oxide of 

 lead and of the acid to which it is united. It consisted of 

 crystals confusedly matted together in a more compact mass 

 than this fossil generally assumes. One hundred grains 

 were dissolved in nitric acid ; the marine acid was separated 

 by nitrate of silver, and any redundant silver by muriate of 

 ammonia. The lead was separated by sulphuric acid, and 

 the superfluous portion of that acid by nitrate of barytes, 

 and the arsenic acid was combined with the oxide of lead 

 by the affusion of nitrate of lead. The muriate of silver 

 = 9*8; the sulphate of lead = 97*6, and the arseniate of 

 lead = 72, equivalent to 1*63 of muriatic acid, 71*46 of 

 oxide of lead, and 23*88 of arsenic acid, respectively. The 

 quartz = 0*35, and the oxide of iron 0*2, nearly. 



Another portion taken from the same specimen, treated 

 with an alkali, gave very nearly a similar result. 



D. 



It will now be necessary for me to speak concerning an 

 ingredient of this fossil, which I may have seemed to over- 

 look. I mean the muriatic acid : I have found some diffi- 

 culty in ascertaining the proportion which it bears to the 

 other constituent parts, and from a cause which I did not 

 suspect. I considered that the only sure mode of deter- 

 mining this point, was to have recourse to nitrate of silver, 

 which might effect a direct separation of the marine acid 

 from the nitric solution of this fossil. But I found, in 

 many experiments upon given quantities of this mineral, 

 that the results, which I derived from this most valuable 

 chemical test, were variable and uncertain. 



At last, I was enabled to trace the error and uncertainly 

 up to two sources. In the first place, I found that the mu- 

 riate of silver was more abundant in the cases where I 

 employed a vessel v^-ith a long neck for the solution, and 

 did not expose it to heat. 



I concluded, therefore, that when the process was con- 

 ducted under different circumstances, the predominating 



mass 



\ 



