A second method consists of covering the pan or boiler at top, and by creating a vacuum within the pan, to favour ebullition and rapid evaporation, at moderate heats. In these very ingenious processes Mr. Howard dispensed with the use of bullocks' blood, and he effected the clarification of the liquor by a system of ingenious canvas filters, assisted by a small quantity of pasty gypsum and alumina, formed by a saturated solution of alum in quick lime. In the last purification, in place of covering the base of the sugar loaves with a stratum of wet clay, he covers them with a stratum of very fine saturated syrup.
In 1815, Messrs. P. and J. Martineau took out a patent for an improved method of refining sugar. It consists in employing animal charcoal, such as ivory black, bone ash or bone black, aluminous earths, ochres and lamp black. They prefer, how ever, the animal charcoal, and they use from two to five pounds of it for every cwt. of sugar to be refined. The usual finings of eggs, blood, or other albu minous matter, are used also in larger quantities than formerly, in order to combine the charcoal, &c. with the dirt in the sugar. The rest of the pro cesses are the same as in common use; but the sugar thus produced has a high degree of purity and whiteness.
The most recent process for purifying sugar, and one which, from an accidental circumstance, has acquired great notoriety, is that of Mr. Daniel Wilson, who conceived the ingenious idea of boil ing sugar by means of heated oil, passing through a worm or coil of copper pipe, fixed within the pan containing the sugar.
A view of this apparatus is annexed.
In the preceding elevation A is a wrought iron vessel for heating the oil, similar to the boiler of a steam engine. It is set in brick work, with a fire under it of a moderate size, and without any flues round the sides, so that the whole action of the fire is upon the bottom. It is made of an oblong form, and its length should exceed its breadth as much as the situation it is to be placed in will allow. The size depends upon the quantity of oil to be heated, or the liquor which is to be evaporated ; and it is observable, that the more the surface presented to the fire exceeds the evaporating surface, the greater will be the economy of fuel. Whale oil, free from sediment, is found to answer better than any other for this purpose, and the quantity necessary to be employed is merely sufficient to cover the bottom of the vessel, to the depth of six or eight inches.
B is a thermometer for ascertaining the heat of the oil.
C is a small tube, opening at the lower end into the oil vessel, while the upper extremity passes into a long flue, called a steam vent, and communica ting with the atmosphere. This pipe serves three different purposes: the first is, that before the pump begins to work in the morning, there is a quantity of air contained in it, and it is necessary there should be a vent for this when the pump is set to work, in order to prevent any compression in the inside of the vessel. The next is, that with a com mon suction-pump it is necessary there should be a communication with the atmosphere. Thirdly, it is designed to carry off' the aqueous vapour from the fresh oil, which has a very bad smell, and such vapours would injure the sugars, if they got abroad in the sugar-house.
D is a cast iron pump, with a spring metallic piston communicating with the oil vessel A, by means of its suction-pipe E. It is set in mo tion in the usual manner, by some mechanical power.
F is a copper vessel, the bottom of which is cov ered in the inside by a coil or pipe, communicating at one of its ends with the pump at G, and at the other end with the oil vessel through the pipe 11. Through this coil of pipe the heated oil circulates, and being surrounded on all sides by the liquid in the pan F, it gives out about 100° of heat in its pas sage, and returns to the oil vessel to obtain a fresh increase of temperature. This pan is surrounded by brick or wood work, to prevent cooling. Of course it has no fire under it.
This ingenious apparatus was erected in August 1819, hy Messrs. Severn, King and Company, and one of the pans was wrought with great satisfaction and profit for nearly three months, when in Nov. 1819, a fire broke out and occasioned a loss of L.80, 000. On the ground that this new mode was more dangerous than the other one, the insurance offices refused to make good the above loss, and that cele brated trial took place in which half of the London chemists gave evidence the reverse of the other half. The point was decided against the insurance offices.