PHOSPHATIC MANURES 207 



supposed. If applied in large quantities to poor land 

 they might be useful as a kind of permanent improve- 

 ment. Smaller quantities of some of the more active 

 manufactured products would, however, give better im- 

 mediate results, and if applied at intervals, would, in the 

 end, be more profitable. The use of untreated native 

 phosphates never became general for the ordinary pur- 

 poses of farming and is now obsolete. Special interest 

 attaches to them as the substances from which super- 

 phosphates and other special manures are chiefly pre- 

 pared, and it is necessary to give some account of them. 

 General Properties. There are several kinds or 

 varieties of native phosphate differing both in origin and 

 in character. They occur in grains, small fragments, 

 and in rock-like masses, in various parts of the world. 

 Some are of organic origin, but the majority are purely 

 mineral. The latter occur both in the crystalline and 

 amorphous forms, and are often weathered and partly 

 decomposed. They are all practically insoluble in pure 

 water, but are slowly attacked by carbonic acid, and are 

 fairly easily dissolved by strong acids. With the excep- 

 tion of isome phosphates of iron and aluminium, which 

 occur in considerable quantities but are not very widely 

 distributed, they all consist essentially of normal ortho- 

 phosphate of lime, called tricalcic phosphate. Some are 

 met with in a very pure state and contain upwards of 

 95 per cent, of tricalcic phosphate, but they are usually 

 associated with larger or smaller quantities of quartz, 

 calcium carbonate, calcium fluoride, oxides of iron, 

 alumina, and other impurities, and the proportion of 

 tricalcic phosphate is sometimes under 30 per cent. 



Production and Imports of Phosphates. The total pro- 

 duction of phosphates in various parts of the world, in- 

 creased from about half a million tons in 1880 to more 

 than a million tons in 1890, and in 1900 it amounted 



