PRESENCE OF ARSENIC. 599 
erusts of the two metals under some circumstan- 
ces, and particularly when they are examined by 
a practised eye. Indeed, Mr. Thompson con- 
cludes his communication by stating, that he fears 
we can only regard Mr. Marsh’s very ingenious 
test as furnishing good collateral evidence, capa- 
ble, in scientific hands, of giving very correct 
indications, but wholly unfit to be entrusted to 
those unaccustomed to careful chemical manipula- 
tion: he says this with a thorough conviction of 
the great utility of the test, and is only sorry that 
its evidence is not unequivocal. 
_ This announcement of Mr. Thompson was the 
cause of a subsequent paper by Mr. Marsh, ap- 
pearing in the Phil. Magazine for Oct., 1839, in 
which he said he was happy in being able to lay 
before the readers of that journal, a very simple 
distinguishing test for arsenic and antimony ; and 
stated it to be as follows : 
After the common arrangements have been 
made for testing for the metals in question, the 
piece of glass or porcelain, on which the metallic 
crusts are generally received, is to have a drop 
of distilled water placed on it; it is then to be 
inverted, so that the drop of water is suspended 
