SECT. in. OF OXALIC ACID. 117 



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 car- 

 bonate 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 re- 

 mains. 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 con- 

 verted 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 sub- 

 stance 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, 



