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: so 
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- 
dingy and so d cheele’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 
per in all these experiments. A source of error so very 
easily discovered, is in fact no source of error at all. 
othe second assertion I object, that the statement in — 
your summary is not quite accurate as a matter of fact. 
sptical: deception that can impose upon no chemist. But, 
‘to a strong solution of sulphat of copper, you add an equal 
guantity of chromat of potash of the strength usually em- 
