305 
CHROMAT OF LEAD, 
Mr. William Hembel, jun. of Philadelphia, having had much 
experience in making the chromat of Lead, I wrote to him for his 
process ; which, with the liberality so common among scientific 
Chymists, he has been good enough to send to me. I have seei\ 
no specimens of the colour equal to what he has made. T. C. 
Dear Sir, 
THE great difficulty in preparing the Chromat of Lead, is to 
preserve an uniform colour during the whole precipitation with 
nitrate of Lead. Vauquelin, Murray and other writers, found the 
difficulty so insurmountable, that, they concluded, Chromat of 
Lead could never be made subservient to the arts. 
In my first experiments, I experienced similar disappoints 
ments ; prepossessed with the opinion of Mr. Vauquelin, I con- 
cluded, that, the different colours of the same precipitation, were 
occasioned by different quantities of alumina, or magnesia, which 
the chromic acid carried down, by an affinity too strong to be in- 
terrupted, otherwise than by the regular order of analysis. 
It will however be obvious to you, that such a proceeding would 
be totally incompatible with the despatch of the manufactory, even 
supposing that the manufacturer possessed the talents to conduct 
such complicated processes, which it is often necessary to modify s 
from causes which can only be appreciated by the practical chy- 
mist. 
Disappointed in every attempt to procure a deep and uniform 
colour, I finally separated all the earths, previous to adding the ni- 
trate of lead ; under the persuasion, that when they were removed 
no obstacle would remain to prevent an artificial product being ob- 
tained equal in beauty to the natural : but the result disappointed 
my expectations, and at the same time instructed me, that it was 
not the earths only, but carbonic acid also, which had hitherto baf- 
fled my endeavours. 
I had long satisfied myself, that no quantity of acid was suffi- 
cient to expel all the carbonic acid from alkalies, in the time allot- 
ted to processes of the laboratory, unless the solution was heated 
to the boiling temperature at the same time ; and reasoning from 
that fact, I concluded, that the carbonic acid, which the alkali of 
the nitre acquired during the fusion of the mineral, combined with 
part of the nitrate of lead, which precipitating with the chromat 
Vol. III. Q q 
I 
