201 
Mr. Gregor on a native Arseniate of Lead. 
sion of ammonia, a greenish matter, which, after ignition, 
became red, and = 0.033 of a grain. It was oxide of iron. 
C. 
1. One hundred grains of larger crystals, some of which 
were hollow, and the surfaces of which were slightly and 
partially covered with silky filaments, treated in the same 
way yielded 95.283 of sulphat of lead equivalent to 69.76 of 
oxide, and 80 of arseniat of lead, which indicates 26,4,0 of ar- 
senic acid. The oxide of iron, in this case, amounted to 
only .05 of a grain, and the residuary silica was in too small 
a quantity to be weighed. 
2. I have endeavoured to decompose this fossil by boiling 
it to dryness in a solution of four times its weight of the purest 
subcarbonat of potash, and exposing the dry mass, for a very 
short time, to a low red heat ; but I found, that only a part of 
the arsenic acid had united to the alkali ; the larger portion 
of it was detected in the nitric solution of the residuum ; but 
the relative proportions of the oxide and the acid, were found 
to correspond almost exactly with the foregoing statement of 
them. 
3. I found also, that carbonat of ammonia precipitated this 
mineral, in an unaltered stale , from its solution in nitric acid : 
as no arsenic acid had united with the precipitant. The solution 
of the nitrat of ammonia was evaporated to dryness, and ex- 
posed to a red heat in a platina crucible ; but nothing was left, 
except a slight trace of oxide of lead. We may infer from 
hence, the absence of both the fixed alkalies. 
4. I found in one specimen only of this fossil any notable 
difference in the relative proportions of the oxide of lead and 
