Note 75 — Of the London Chimney-Sweepers. (continued 6)

“It is in the upper basins,” adds the Reports, “that the first separation of the liquids and solids takes place, the latter falling to the bottom, and the former gradually flowing off through a sluice into the lower basins. This first separation, however, is by no means complete, a considerable deposit taking place in the lower basins. The mass in the upper basins, after three or four years, then appears like a thick mud, half liquid, half solid; it is of depth varying from 12 to 15 feet. In order entirely to get rid of the liquids, deep channels are then cut across the mass, by which they are drained off, when the deposit soon becomes sufficiently stiff to permit of its being dug out and spread upon the drying-ground, where, to assist the desiccation, it is turned over two or three times a-day by means of a harrow drawn by a horse.

“The time necessary for the requisite desiccation varies a good deal, according to the season of the year, the temperature, and the dry or moist state of the atmosphere. Ere yet it is entirely deprived of humidity, the matter is collected into heaps, varying in size usually from 8 to 10 yards high, and from 60 to 80 yards long, by 25 or 30 yards wide. These heaps or mounds generally remain a twelvemonth untouched, sometimes even for two or three years; but as fast as the material is required, they are worked from one of the sides by means of pickaxes, shovels, and rakes; the pieces separated are then easily broken and reduced to powder, foreign substances being carefully excluded. This operation, which is the last the matter undergoes, is performed by women. The poudrette then appears like a mould of a grey-black colour, light, greasy to the touch, finely grained, and giving out a particular faint and nauseous odour.

“The finer particles of matter carried by the liquids into the lower basins, and there more gradually deposited in combination with a precipitate from the urine, yield a variety of poudrette, preferred, by the farmers, for its superior fertilizing properties. In this case the drying process is conducted more slowly and with more difficulty than in the other, but more completely.

“In general the poudrette is dried with great difficulty; it appears to have an extreme affinity for water; few substances give out moisture more slowly, or absorb it more greedily from the air.

“A good deal of heat is generated in the heaps of desiccated matter. This is always sensible to the touch, and sometimes results in spontaneous combustion.

“The intensity of this heat is not in proportion to the elevation of temperature of the atmosphere. It is promoted by moisture. The only means of extinguishing the fire when it is once developed is to turn over the mass from top to bottom, in order to expose it to the air. Water thrown upon it, unless in very large quantities, would only increase its activity.”