PREPARATION OF CALOMEL. 
91 
tube contains from ten to fourteen pounds troy of calomel. 
The tubes which I use have been made with the clay em- 
ployed in the manufacture of Paris crucibles. I have gone 
to the pains of having them coated on the outside with a 
layer of another kind of clay. By this means they so far 
resist the action of the fire as to admit of being used several 
times. 
The tube containing the calomel is placed in a furnace, 
the form of which is adapted to it, the open extremity of 
the tube protruding through an aperture made for that pur- 
pose, and communicating with a receiver. The receiver 
which I have hitherto employed is a large stone vase, with 
a hole pierced in the side to receive the open end of the 
tube. A small opening at the top, over which a plate of 
glass is laid, serves to allow of the escape of the diluted 
air. 
Instead of this vessel, a small chamber may be built of 
brick-work attached to the furnace; but for operating on a 
few pounds, I prefer to use the vessel I have described. 
The receiver ought to be placed as near as possible to the 
furnace, to prevent the calomel from condensing in the end 
of the tube. On the other hand, the receiver should be 
protected from the heat of the furnace: to effect this, the 
opening, at which the tube issues from the furnace, should 
be luted with clay, and two polished plates of metal should 
be placed between the furnace and the receiver to prevent 
the radiation of heat. Thus are accomplished the two con- 
ditions essential to success; the tube is heated to within a 
very short distance of its entrance into the receiver, by 
which means the calomel is prevented from condensing in 
the tube; and the receiver is preserved from the heat of 
the furnace, which, by raising the temperature of the air 
too high, would cause the calomel to be deposited in 
crystallized agglomerations instead of a finely-divided 
powder. 
Nothing is more simple than the management of the fire. 
