Advantage is taken of this fact to carry out a process of incineration on a large scale, so that heat derived from the burning off of the resinous mass is utilised for evaporation of weaker liquors. The ash is drawn from special furnaces, put aside, and allowed to char quietly, so that the carbonaceous matter is more or less completely burnt away. The ash in this form contains about 40 per cent. of soda, its composition being determined by the nature of the fibre which has been treated. In the case of straw, the amount of silicate is considerable, as shown by the following typical analysis:-- Sodium carbonate 70·2 Sodium hydrate 2·3 Sodium sulphate 4·1 Sodium chloride 7·5 Silica 7·5 Oxides of iron and alumina 0·75 Unburnt carbon, etc. 7·65 ------ 100·00 ------ At the present time there is no process in general use for the recovery of the liquors used in the treatment of wood by the sulphite process. Many schemes have been proposed, the most promising of which is that of Drewsen. _Sulphur and Sulphites._--The pale yellow brittle substance known as sulphur is too familiar to require any detailed description. It unites with oxygen in various proportions, and these in contact with water form the various sulphur acids known to commerce. Sulphur burned with a limited quantity of air forms sulphurous acid gas, and this substance is the chief product of oxidation, which by further treatment can be converted into sulphites. In the manufacture of the sulphur compounds required in the preparation of wood pulp, the furnace for burning the sulphur consists of a flat-bottomed cast iron retort which is very shallow, and provided with a curved top, to which a pipe is fixed, so that the sulphurous acid may be conveyed away from the furnace. In the most recent form of sulphur oven a small conical-shaped revolving furnace is employed, which produces a satisfactory gas of constant composition very economically. _Bisulphite of Lime._--This compound is obtained when the sulphurous acid gas is brought into contact with moistened limestone. In the manufacture of bisulphite of lime on a large scale the sulphurous acid gas is drawn or pumped up tall circular towers filled with blocks of limestone, kept moistened by a carefully regulated stream of water flowing from the top of the tower. In another system known as the acid tank process, the gas is forced into large circular vats containing milk of lime.