156 On Tests for the Discovery of Arsenic. 
acid, by Mr. Hume and Dr. Marcet, may be produced by 
substituting chromat of potash for the arsenical solution: se 
that this test also is fallacious. The subject is of importance, 
and it is well that any mistakes or oversights to which all of 
us are liable, should be corrected. I have carefully repeat- 
edall Dr. Porter’s experiments, since I saw your summary of 
them; and I should have been glad to have repeated them 
with Dr. Porter himself, applying not only the other tests of 
arsenic, but also my own test of chromat of potash, which I 
do not find that Dr. Porter has noticed among the methods 
of discovering the poison in question. I should have been 
much gratified to have been made acquainted with the am- 
biguities which occurred to Dr. Porter, with his dissertation 
on the subject ; for in such case, both his experiments and 
my own, might have been made more satisfactory. 
To the 1st class of Dr. Porter’s cases, I object (a) that 
although a greenish colour may be produced by onion juice 
and by coffee, with a solution of sulphat of copper, it is so 
dingy and so different from Scheele’s green, and from chro- 
mat of copper, thatthe most inexperienced eye need be under 
no mistake. (b) That Dr. Bostock’s proposal of Scheele’s 
green asa test of arsenic, has never been relied on as deci- 
sive by any other chemist of authority. (c) That it is ut- 
terly impossible for any chemist to be led away by the am- 
biguity of colour produced by Dr. Porter’s reagents, as a 
single drop of ammonia will instantaneously detect the cop- 
per in all these experiments. A source of error so very 
easily discovered, is in fact no source of error at all. 
To the second assertion I object, that the statement in 
your summary is not quite accurate as a matter of fact. 
Take sulphat of copper, precipitate by carbonat of potash, 
add chromat of potash in the way you cite the experiment; 
a green colour will be produced exactly like Scheele’s 
green: so it will if you add infusion of turmeric or gamboge 
or saffron, or any other liquor equally yellow with the chro- 
mat of potash : but this superinduced yellow colour can be 
washed away completely, and the carbonat of copper will 
resume its original blue tint; no chromat of copper is form- 
ed, no chemical union has taken place ; the colour is a mere 
optical deception that can impose upon no chemist. But, 
if to a strong solution of sulphat of copper, you add an equal 
quantity of chromat of potash of the strength usually em- 
