FURNACES. 
91 
muffle-door, is made an opening, c, to be used as a feeding-door ; it has a 
stopper fitted to it, and is made as large as it well can be, to allow the 
more fuel to be supplied to the furnace at each time of opening it. 
All the preceding parts are made of an apyrous clay, brought from 
Vaugirard, and are two inches thick. 
The vent at the top of the dome is nearly eight inches in diameter, 
and has a stone-ware chimney, e, adapted to it, which is two feet high, 
six inches diameter internally, and an inch thick. This chimney is usually 
lengthened by an iron pipe of the same diameter, and twelve feet high. 
The muffles used in this furnace are eight or nine inches long, se 
micircular, with a radius of an inch and half, close on all sides except 
the front, and from one line and a half to two lines thick ; ten small cru 
cibles may be placed in them. 
In the Memoires for 1767, Dr. Macquer relates some ex 
periments made with a furnace of this kind, but two inches 
larger every way, which are very interesting, on account of 
their shewing the differences of effect produced by altering 
the length and diameter of the chimney. 
When this larger furnace, whose fire-room was, of course, 
about fifteen inches from front to back, and thirteen wide, 
had a chimney adapted to it, of six inches diameter, and 
eight feet in length, the furnace consumed a voie, or about 
130 pounds of charcoal in an hour, roared so that the noise 
resembled that of a coach rattling over a bridge, and all the 
glasses and other things in the laboratory were strongly 
shaken. 
This fire being continued for three hours, the following 
substances, which had been exposed to the fire, were found 
to be thus altered :—1. A Norwegian stone, resembling 
Briançon, or French chalk, was merely hardenéd external 
ly. 2. Unwashed white clay, and the same washed, were 
only hardened, and shewed no signs of melting. 3. A hard 
crystalline substance from Alençon, was entirely melted 
into a white milky glass. 4. Gypsum was melted. 5. 
Calx of tin, prepared by nitric acid, was changed to a red 
colour, and had began to melt. These substances were 
chosen for experiment, because Mr. Pott had found them to 
resist all his efforts to melt them. 
When the chimney was lengthened to fourteen feet, the 
effects were inferior, although the firing was continued for 
seven hours. 
When sixteen feet of chimney, eight inches in diameter, 
were used, and the fire was continued for three hours and a 
half, the effects were superior to those of the last experi 
ments. 
The effects of the fire were fully equal to those obtained 
by Mr. Darcet, in the Count de Lauraguais’ porcelain fur¬