When sawdust, mixed with two equivalents of the hydrate of soda and one equivalent of the hydrate of potash, is exposed to a heat of 400° for a few hours, the substances are fused, and when raised to a still higher temperature the hydrates are decomposed: hydrogen is evolved, and the carbon combines with the oxygen to form the oxalate of soda and the oxalate of potash. In order to separate these oxalates they are put into a filter, a solution of carbonate of soda is passed through it; the oxalate of soda remains in the filter, the carbonate of potash passes through it; and when lime is added to the oxalate of soda, the soda is liberated, passes through the filter, and the oxalate of lime remains. Sulphuric acid is then added to the oxalate of lime, sulphate of lime is formed, and oxalic acid mixed with water remains, and by evaporation forms into beautiful crystals of oxalic acid. This is an instance of a complicated chemical process; nevertheless it is carried on to a vast extent in Manchester, nine tons a week being furnished by one manufactory alone. Two pounds of sawdust yield one pound of oxalic acid. In ordinary distillation a volatile substance such as water, by absorbing the heat applied to it, becomes converted into vapour; by abstracting the absorbed heat from the vapour, it is reconverted into the original substance. Destructive distillation, on the contrary, consists of an entire destruction of the original substance and a simultaneous production of new substances. Of this the destructive distillation of coal furnishes the most interesting illustration, and shows at the same time the success of modern chemistry in utilizing waste substances. Coal had been distilled for years to furnish gas for the illumination of our cities before it was discovered that the refuse contained principles of the greatest value. The products of the distillation are threefold: gas, coal water, and coal tar. Coal gas is a combination of various gases, whose illuminating properties depend upon, and are exactly in proportion to, the quantity of carbon they contain. The particles of carbon raised to a white heat give the light, for the gaseous part has a feeble flame, and requires a higher temperature than solid matter, which becomes luminous at about 700° in the dark, and at from 1000° to 2000° in bright daylight. Coal gas consists of a combination of illuminants: olefiant gas, which contains 86 per cent. of carbon, carburetted hydrogen or marsh gas, which contains 75 per cent., carbonic oxide, carbonic acid gas, hydrogen, sulphuretted hydrogen, and a very small quantity of nitrogen, besides the bisulphide of carbon, and benzol, a pure hydro-carbon, consisting of 12 equivalents of carbon and 6 of hydrogen.