46ft 
Prussian Blue, 
to be carried on there. In England, these articles are 3 
or 4 times the price, and the alkali also is much dearer. 
The actual process in Philadelphia is not essentially differ- 
ent from those above described : the hoofs are calcined in 
fcn iron pot : potash is added and calcined in a strong heat 
with the charcoal of the hoofs : the mixture is washed in 
hot water, and a mingled solution of green copperas and 
alum is gradually added. The dirty- coloured mass, is 
cleared by digesting it with dilute vitriolic or marine acids, 
of which I am persuaded the latter is the best. The actu- 
al proportions, as it was improper for me to enquire, I am 
unable to communicate. My own opinion is, that one 
part and a half of alkali to one part of animal charcoal 
(that is after the blood, the hoofs, the horns, and the lea- 
ther are charred) and one part and a half of green vitriol 
and as much alum, are enough for a fine colour. The 
colour is never perfect till after a few days exposure to 
the air. 
Experiments are wanting to shew, 1st. What proportion 
of potash, the animal charcoal can saturate. 21y. What 
proportion of sulphat of iron, the prussian alkali can pre- 
cipitate. 31y. Whether the prussian alkali throws down 
the iron or the earth of alum in preference, and in what 
proportion. 41y. Whether the alum contributes to dis- 
solve the yellow oxyd of iron. 
It appears to me, that alum has the following uses : it 
is thrown dow r n, not by the prussiat but by the carbonat 
of potash, and therefore prevents much of the yellow oxyd 
of iron from being precipitated : it adds to the body and to 
the brilliancy, and to the cheapness of the colour, but not 
to the depth or intensity of it, 
