"If the air rushes out with too much rapidity, and with a hissing noise, the door of the ash-hole must be entirely shut, in order to slacken the fire. If it do not strike pretty smartly against the hand, that door must be opened wider, and large coals thrown into the fire-place to quicken the fire immediately.
"The operation usually lasts four and twenty hours; and the following signs shew that it will succeed, provided the retort resist the fire.
"You must begin the operation with putting some unlighted charcoal in the ash-hole, and a little lighted charcoal at the door thereof, in order to warm the retort very slowly. When the whole is kindled, push it into the ash-hole, and close the door thereof with a tile. This moderate heat brings over the phlegm of the mixture. The same degree of fire must be kept up four hours, after which some coals may be laid on the grate of the fire-place, which the fire underneath will kindle by degrees. With this second heat brought nearer the retort, the ballon grows warm, and is filled with white vapours, which have the smell of fetid oil. In four hours after, this vessel will grow cool and clear; and then you must open the door of the ash-hole one inch, throw fresh coals into the fire-place every three minutes, and every time shut the door of it, lest the cold air from without should strike against the bottom of the retort and crack it.
"When the fire has been kept up to this degree for about two hours, the inside of the ballon begins to be netted over with a volatile salt of a singular nature, which cannot be driven up but by a very violent fire, and which smells pretty strong of peach-kernels. Care must be taken that this concrete salt do not stop the little hole in the ballon: for in that case it would burst, the retort being then red-hot, and the air exceedingly rarefied. The water in the ballon, being heated by the vicinity of the furnace, exhales vapours which dissolve this sprigged salt, and the ballon clears up in half an hour after it has ceased rising.
"In about three hours from the first appearance of this salt, the ballon is again filled with new vapours, which smell like Sal Ammoniac thrown upon burning coals. They condense on the sides of the receiver into a salt which is not branched like the former, but appears in long perpendicular streaks, which the vapours of the water do not dissolve. These white vapours are the fore-runners of the Phosphorus, and a little before they cease to rise they lose their first smell of Sal Ammoniac, and acquire the odour of garlic.
"As they ascend with great rapidity, the little hole must be frequently opened, to observe whether the hissing be not too strong: for, in that case, it would be necessary to shut the door of the ash-hole quite close. These white vapours continue two hours. When you find they cease rising, make a small passage through the dome, by opening some of its registers, that the flame may just begin to draw. Keep up the fire in this mean state till the first volatile Phosphorus begin to appear.
"This appears in about three hours after the white vapours first begin to rise. In order to discover it, pull out the little birchen peg once every minute, and rub it against some hot part of the furnace, where it will leave a trail of light, if there be any Phosphorus upon it.
"Soon after you observe this sign, there will issue out through the little hole of the ballon a stream of blueish light, which continues of a greater or shorter extent to the end of the operation. This stream or spout of light does not burn. If you hold your finger against it for twenty or thirty seconds, the light will adhere to it; and if you rub that finger over your hand, the light will besmear it, and render it luminous.
"But from time to time this streamer darts out to the length of seven or eight inches, snapping and emitting sparks of fire; and then it burns all combustible bodies that come in its way. When you observe this, you must manage the fire very warily, and shut the door of the ash-hole quite close, yet without ceasing to throw coals into the fire-place every two minutes.
"The Volatile Phosphorus continues two hours; after which the little spout of light contracts to the length of a line or two: and now is the time for pushing your fire to the utmost: immediately set the door of the ash-hole wide open, throw billets of wood into it, unstop all the registers of the reverberatory, supply the fire-place with large coals every minute: in short, for six or seven hours all the inside of the furnace must be kept of a white heat, so that the retort shall not be distinguishable.