202 
Mr. Gregor cn a native Arseniate of Lead 
of the acid, to which it is united. It consisted of crystals con- 
fusedly 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 nitrat of silver, 
and any redundant silver by muriat of ammonia. The lead 
was separated by sulphuric acid, and the superfluous portion 
of that acid by nitrat of barytes, and the arsenic acid was com- 
bined with the oxide of lead by the affusion of nitrat of lead. 
The muriat of silver = 9,8 ; the sulphat of lead = 97.6, and 
the arseniat of lead — 72, equivalent to 1.63 of muriatic acid, 
7 3,4 6 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 in- 
gredient of this fossil, which I may have seemed to overlook. 
I mean the muriatic acid : I have found some difficulty in as- 
certaining the proportion which it bears to the other consti- 
tuent parts, and from a cause, which I did not suspect. I 
considered that the only sure mode of determining this point, 
was to have recourse to nitrat 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 un- 
certain. 
At last, I was enabled to trace the error and uncertainty up 
to two sources. In the first place, I found that the muriat of 
