On Tesis for the Discovery of Arsenic. 157 
ployed by the manufacturers of chromic yellow, you will 
produce a green chromat of copper, that will retain its col- 
our unchanged by repeated ablution. Chromat of potash 
therefore cannot be substituted for arsenic, if the experiment 
be made as Dr. Porter has directed. I acknowledge, how- 
ever, that I should have remained ignorant of the green col- 
our produced by the admixture of sulphat of copper and 
chromat of potash, if Dr. Porter’s experiments had not led 
me to it. 
To the third statement of Dr. Porter, I object, that it is 
not conformable to my experience. Take a solution of 
arsenious acid, or of arsenite of potash, dip a slender glass 
rod in a solution of nitrat of silver, and another in ammonia, 
bring them near together, and immerse them in the arsenic- 
al solution as Dr. Marcet directs ; in this case, a dense yel- 
low precipitate will appear. In lieu of nitrat of silver take 
chromat of potash as Dr. Porter directs ; the result wedl not 
be the same ; a yellow tint will be given to the liquor, owing 
to the yellow colour of the chromat of potash, but no pre- 
cipitate will take place. If the arsenical solution be strong, 
and the chromat of potash be added in equal quantity or 
somewhat more, the yellow tint will in half an hour become 
green, Owing to the arsenious acid robbing the chroinic acid 
of a part of its oxygen ; the arsenious being converted into 
the arsenic, and the chromic acid into the chromic oxyd. 
But I aver that it is utterly impossible to confound the dense 
yellow precipitate produced in Dr. Marcet’s experiment, 
with any of the appearances produced by Dr. Porter’s sub- 
stitute of chromat of potash. Again: take chromat of pot- 
ash diluted or not diluted as you please. Operate on this 
chromat of potash as Dr. Marcet directs you to operate on 
the arsenical solution ; that is, dip a glass rod in a solution 
of nitrat of silver,*and another in ammonia, bring them near 
together, and immerse them in the solution of chromat of 
potash as you would in an arsenical solution—the precipitate 
will assume the drown tinge of chromat of silver, perfectly 
different from the yellow arsenite of silver. Dr. Porter 
may have a method of performing the experiment not sug- 
gested in your summary, but if it be intended to form an ob- 
jection to Dr. Marcet’s test, Dr. Marcet’s process should be 
followed. 
