160 On Tests 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, I 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- 
fore it appeared in their transactions. I didso in my treatise 
on Medical Jurisprudence published in 1819. I appli- 
ed 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 in 
your last number, gave me the first information that that 
eentleman had been pursuing similar researches. I shall, 
however rejoice, if among us, some useful truths are elicited 
on a subject of great interest. As you have not probably 
seen the publications to which [ allude, I shall state a sum- 
mary of the experiments that will enable your readers to de- 
tect arsenic by means of chrome. Take five watch glasses, 
x 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 either 
aqueous or alcoholic ; on the fifth, two or three drops of a 
solution of copper ; add to each two or three or four drops 
of chromat of potash of the usual manufacturer’s strength. 
In half an 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 
adrop of ammonia will instantly convert to a blue, The 
acid of chrome is converted into the green oxyd, by the 
arsenious acid, which is now the arsenic acid. 
I am, dear Sir, your friend and servant, 
THOMAS COOPER. 
Dr. Porter’s reply to Dr. Cooper’s remarks. 
New-Haven, Sept. 4, 1821. 
It may be premised that my experiments and remarks 
were not made with any view to publication, or they would 
