known by the Name of Ivory Black, &c. 19 
iron rods a little longer than the cylinder. Assoon as the combus- 
tion is over, by drawing out the rods the disk at once carries 
down all the coal into an extinguisher adapted to the mouth of 
the cylinder. The disk is then pushed down, and the cylinder is 
charged and stopped again before it has time to cool. The time 
saved by this method ceconomizes a considerable quantity of fuel. 
It is, however, very essential that the cylinders or cucurbits 
receive the heat equally every where, and that depends on the 
construction of the furnace. This is a difficult problem to re- 
solve: however, many coal-distillers have effected it, by rendering 
their cylinders or cucurbits moveable, so as to be able to turn 
them four or five times during the operation, and present every 
side to the full force of the fire alternately. 
In Monsieur Robert’s manufactory at the Gros Caillou, where 
they extract oil from garbage, Monsieur Barruel, head chemist of 
the School of Physic, got a great furnace constructed for distilling 
bones, in which the laws of pyrotechny are so strictly adhered 
to, that he can heat his cylinders with cow-dung only, and com- 
pletes the distillation by consuming the gas in the body of the fur- 
nace. His cylinders are always hot; and the operations succeed 
so rapidly, that the expense of fuel is hardly sensible. 
Every kind of bones employed in close vessels does not yield 
a similar kind of coal; this coal varies in quality, according as 
they employ old or young animals’ bones, round or flat, heavy 
and compact, or spongy and light ones. The analysis of these 
various kinds of charcoal has made us acquainted with the cause 
of this difference. It was natural to think that young animals’ 
bones contained more gelatine than those of old quadrupeds ; 
and, consequently, ought to yield a deeper black and more char- 
coal. This was an error; for great round bones, such as the fe- 
mur and tibia of oxen, yield more coal when distilled than similar 
bones of equal weight taken from calves. The proportion of black 
charcoal in young animals’ bones is only four or five per cent., 
while that of old compact bone amounts to forty. This is the 
reason why ivory black is the most intense of all animal blacks. 
Animal charcoal is a mixture of phosphate of lime, a small 
quantity of quick-lime, and coal (or calx of carbon). The pro- 
perty of clarifying liquids depends on the mixture of these four 
substances, none of which separately enjoys this property so per- 
fectly. Now, as all manufacturers are in the habit of deciding 
on the quality of their materials, —when the bones do not appear 
to contain much gelatine, they take care to add, in the furnace, 
soft animal matter, such as clotted blood, tripe or guts, mem- 
branes, &c. This is the reason why many refiners esteem most 
the black produced by the calcination of blood and potash, in 
Prussian-blue manufactories. 
From 
