Lead. 
265 
strong red heat with three fourths of its weight of clean 
pure nitre, the nitric acid is decomposed, the chrome is 
acidified, and then combines with the alkaline base of the 
nitre, forming a chromat of potash. To obtain this pure, 
the solution must be repeatedly crystallized to get rid 
of a small portion of iron, which will gradually fall down 
in an oxyd ; the other impurities, such as the earths that 
might have been taken up in small quantities, will also 
fall down. The solution after two or three crystalliza- 
tions will then contain nothing but chromat of potash, to- 
gether with a small quantity of nitric, or uncombined 
alkali ; this can be ascertained by an intermixture of white 
with the yellow precipitate of lead ; and can be obviated 
by a small quantity of nitric acid. When the chromat of 
potash is concentrated, and added to a solution of nitrat 
or even acetat of lead, (which need not be religiously satu- 
rated with metal) a yellow precipitate will fall down, 
which is the yellow chromat of lead. The chromat of 
potash will throw down nitrat of mercury a deep orange 
colour. 
Naples Yellow . This ought to be a natural production 
found near Naples ; a kind of lava, unchangeable by fire 
and by acids. The artificial kind is thus made. Take 
12 ounces of pure white lead, and 2 ounces of washed 
calx of antimony ; the common diaphoretic antimony of 
the shops ; half an ounce of calcined Roman alum, which 
is generally free from any admixture of iron, and one 
ounce of sal ammoniac. Pound them well together, and 
put them in an earthen crucible with a cover. Keep 
them in a low red heat for three hours. If you want, the 
mixture more of a gold colour, add a quarter of an ounce 
more of antimony, and as much sal ammoniac. Do not let 
any iron touch this colour. 
Vox. Ill, L \ 
