204 Mr. Gregor on a native Arseniate of Lead. 
on charcoal and ignited, it was totally dissipated in arsenical 
fumes. 
Some of it was dissolved in water, and dropped into liquid 
sulphat of titanium, a white precipitate was produced: com- 
bined with soda, it precipitated silver from the nitrat of silver, 
of a brick colour. It precipitated mercury from its nitrat, of 
a yellowish colour, which afterwards became reddish. This 
precipitate exposed to the flame of the blow-pipe on charcoal, 
exhibited the same phenomena as arseniat of mercury. 
I precipitated magnesia from its muriat, and redissolved it 
by carbonat of ammonia, perfectly saturated with carbonic 
acid. I divided this liquid into two portions, and dropped into 
both a solution of the combination of the acid of this mineral 
and soda. No precipitate was produced. I dropped into one 
of the vessels some liquid phosphat of soda, and a separation 
of saline matter was instantly produced. I soon, however, 
found, that this mode of distinguishing the phosphoric from 
the arsenic acid could not be depended upon. For in the 
other vessel, in which no phosphat of soda had been dropped, 
in a short time, saline tufts made their appearance, and an 
abundant deposition of saline matter was formed. I found 
also, that if the solution had been more concentrated, the pre- 
cipitation would have immediately taken place. 
On making a comparative experiment with arsenic acid, I 
found that it forms a triple salt with ammonia and magnesia, 
analogous to the phosphoric salt described by Dr. Wollaston. 
The figure of the arsenical salt, as far as I could determine it 
from a confused crystallization, is a triedral prism. 
We are therefore, I think, authorized from the experiments 
herein detailed, to conclude, that the fossil which is the subject 
