Coal contains on an average from a half to one and a half per cent of nitrogen. When it is subjected to dry distillation, as is done in the gas-works, the nitrogen which it contains is chiefly converted into ammonia, and, in the process of purification of the gas, is removed in the "gas-liquor,"[213] which contains about one per cent of ammonia. The ammonia recovered from this liquor by distillation is then absorbed in sulphuric acid. It may be pointed out that nothing like all the nitrogen contained in the coal is recovered as sulphate of ammonia. It has been calculated that only from a fifth to a tenth is actually recovered, and many processes have been patented with a view to increasing the yield of ammonia in gas manufacture. The total production of ammonia from gas-works may be placed at little over 100,000 tons per annum for Great Britain. Mr L. Mond, F.R.S., recently drew attention to the possibility of largely increasing our supply of sulphate of ammonia from coal. As indicating what an enormous source of sulphate of ammonia we have in coal, Mr Mond calculated that its annual consumption in this country (estimated at 150,000,000 tons) would yield as much as 5,000,000 tons of sulphate of ammonia. _Other Sources._ While the ammonia produced in the manufacture of gas has long been collected, it is only of recent years that the other sources of ammonia have been developed. Next to the gas-works, the shale-works of Scotland form in this country the chief source of this valuable manure. In these works the ammonia is obtained in distilling the paraffin shale by a method somewhat similar to that in use in the gas-works. The amount of sulphate of ammonia obtained from this source is between 20,000 and 30,000 tons per annum. Recently the ammonia has been recovered from the blast-furnace gases in iron-works--some 6000 tons being annually obtained in this way; while from coke and carbonising works the annual production is about half that amount. The combined annual production from all these sources may be put down at 140,000 tons, the total production in Europe being probably little more than 200,000 tons. In the Appendix further statistics will be found.[214] _Composition, &c., of Sulphate of Ammonia._