160 On Tesis for the Discovery of Arsenic. 
The use of chromat of potash as a test of arsenic, I first 
ascertained and published in the summer of 1818, among 
the additions | made to Accum’s chemical amusements. 
In September of the same year, | read a memoir on the 
subject before the American Philosophical Society, and ex- 
hibited the experiments to the members then present. J 
was permitted to publish the substance of that memoir, be- 
oa it appeared in their transactions. Ididso in my treatise 
n Medical Jurisprudence published in 1819. I appli- 
a it among other tests, with the most perfect success, last 
winter, to the contents of the stomach of a man suspected of 
being poisoned by arsenic, before my class here, and Dr. 
Davis and his students. 
The summary of Dr. Porter’s experiments contained i in 
your last number, gave me the first information that that 
gentleman had been pursuing similar researches. I shall, 
however sees if among us, some useful truths are elicited 
on a subject of great interest. As_ you. have not probably 
seen the pu ti cation to which I allude, I shall state a sum- 
‘mary experiments that will enable: your readers to de- 
pieanesuie bY aiid ofchrome. Take five watch glasses, 
or the bottoms of as many wine glasses, or tumblers. Put 
on one, two or three drops of a solution of white arsenic ; 
on the ‘second, as much arseniat of potash ; on the third, 
one fourth of a grain of white arsenic in substance ; on the 
fourth, two or three drops of solution of sublimate sae 
aqueous or alcoholic ; on the fifth, two or three drops 0 
solution of copper ; add to each two or three or four icp 
of chromat of potash of the usual manufacturer’s strength. 
In half hour a bright, clear, grass-green colour will appear 
in’Nos. 1, 2, 3, unchangeable by ammonia. No. 4 will 
instantly exhibit an orange precipitate: No. 5a green, which 
arsenious acid, which is now the arsenic acid. 
am, dear Sir, your friend and servant, 
THOMAS COOPER. 
Dr. Porter's reply to 5 Be ‘Cisopi remarks. 
Ne sii Sept. 4, 1821. 
= may be premised that my ex and 
were not made with any view to publication, or they met 
