200 ECONOMIC GEOLOGY

indispensable for high explosives most nations will probably
manufacture them, so as to be independent of an imported
supply.

PuospuATES 1

VaLuE anD Use oF ProspaaTes—Phosphate of lime is a
much scarcer mineral than carbonate of lime; it is of high
importance as a fertilizer, being especially useful in the
growth of grain, Burnt bones had long been used in China
as manure, and their value is explained by Gahn's discovery
 in 1769 that bones contain phosphorous; but as bone
phosphate is insoluble the fertilizing effect of bones was
attributed to their gelatine, until in 1843 experiments by
the Duke of Richmond showed that ground bones with and
without their gelatine were equal in fertilizing value. Normal
phosphate of lime (CayP,0q) is insoluble; but it was discovered
 by Lawes in England and by Liebig in Germany,
that the action of sulphuric acid on bones and mineral phosphate
 converts them into superphosphate or acid phosphate
(CaH4(POy2, H,0) which is soluble and usable as plant
food. The exhaustion of many wheat fields in North-western
Europe about that date led to fears of famine. Beds of
phosphatic nodules occur in the S.E. of England, and their
conversion into superphosphate rendered them available
for the refertilization of the European grain-fields. Emerson,
the American philosopher, suggested that spendthrift agriculture
 might be saved by agricultural chemistry * offering
by a teaspoonful of artificial guano to turn a sandbank into
corn; while Liebig, with generous enthusiasm, declared
that England had in its mineral phosphates, a source of
wealth equal to its coal-fields. The supply of English phosphate
 was disappointingly small, and the industry became
dependent upon imports from warm temperate and tropical
cegions.
The commercial phosphates are due to the concentration
of phosphoric acid by various processes (Fig. 53).
The primary source of the phosphorus is the apatite in
igneous rocks. It is a tricalcic phosphate of lime (Ca,P,04),
LA general account of the geology of phosphates has been given by
che author, 77. G. Soc. Glasgow, xvi, 1917, pp. 116-63.