Year. Tons. | Year. Tons. 1873 225,000 | 1873 124,000 1874 230,000 | 1874 108,200 1875 280,000 | 1875 164,900 1876 300,000 | 1876 166,800 1877 208,000 | 1877 69,600 1878 250,000 | 1878 104,400 1879 205,000 | 1879 55,300 1880 140,000 | 1880 48,300 1881 230,000 | 1881 54,800 1882 335,000 | 1882 96,000 1883 440,000 | 1883 103,700 1884 505,000 | 1884 103,700 1885 380,000 | 1885 109,400 1886 330,000 | 1886 75,100 1887 440,000 | 1887 83,100 1888 640,000 | 1888 103,100 1889 760,000 | 1889 120,000 1890 784,000 | 1890 114,000 1891 851,000 | 1891 121,000 1892 795,000 | 1892 115,000 CHAPTER X. SULPHATE OF AMMONIA. _Value of Ammonia as a Manure._ The value of ammonia salts as a manure has been long recognised; indeed till recently ammonia was thought to be the most valuable form in which nitrogen could be applied as a plant-food--a view, we may mention, held by Liebig. While the plant, no doubt, can absorb its nitrogen in the form of ammonia,[212] as well as in other forms, as we have already pointed out in previous chapters, it is now fully recognised that ammonia salts, when applied to the soil, are converted into nitrates. Nitric acid, then, must be regarded as the most valuable, inasmuch as it is the most rapidly assimilated form of nitrogen for the plant; but next to nitric acid in value comes ammonia. Of the different forms of ammonia available for manurial purposes, the only one used to a large extent is sulphate. _Sources of Sulphate of Ammonia._ The oldest, and what is still the chief source of this valuable salt, is the gas-works, where it is obtained as one of the bye-products in the manufacture of gas. It is also obtained to a lesser extent from shale, iron, coke, and carbonising works. Bones, horn, leather, and certain other animal substances rich in nitrogen, when subjected to dry distillation, as is the case in certain manufactures, such as the manufacture of bone-charcoal for use in sugar-refineries, and the distillation of horn, &c., in the manufacture of prussiate of potash, also constitute less abundant sources. _Ammonia from Gas-works._