434 Composition of Phosphatic Minerals used in Agriculture. 
are procurable at a trifling expense, they may be used with advan- 
tage in a powdered state, like marl, in large quantities, say at 
the rate of a ton or more per acre ; but as a rule, phosphatic 
minerals are mainly of use to the manufacturer of superphos- 
phate, whose aim it should be to render, by treatment Avith sul- 
phuric acid, the insoluble phosphate of lime contained in them 
as completely soluble in water as possible. 
Soluble phosphate of lime is a definite chemical compound, 
and in all respects just as valuable for manuring purposes as 
soluble phosphate obtained from bones. On the other hand, 
insoluble phosphate, in the shape of undecomposed phosphatic 
minerals, has little or no practicable manuring value, whilst in 
the shape of bone-dust it is sufficiently available as plant-food 
to be of considerable value. 
In conclusion, I shall briefly revert to the various conditions 
which affect the commercial value of phosphatic minerals as 
raw materials for the manufacture of superphosphate and com- 
pound artificial manures. 
The commercial value of phosphatic minerals is obviously 
regulated by the percentage of phosphate of lime they contain. 
The richer they are in this element, I need hardly say, the more 
valuable the^'' are, other circumstances being the same, to the 
manufacturers of artificial manures. 
But the percentage of phosphate of lime alone is not the 
sole measure of their commercial value ; and it not infre- 
quently happens that a phosphate having a lower percentage 
of phosphate of lime, nevertheless is worth more, weight for 
weight, than another kind richer in phosphate of lime ; for 
whilst some impurities, for instance, quartz rock and insoluble 
siliceous matters, do not consume any sulphuric acid, others, 
like carbonate of lime, neutralise or render ineffective a portion 
of the acid which is employed in the manufacture of superphos- 
phate for rendering the phosphate soluble. Any excessive pro- 
portion of carbonate of lime in a mineral phosphate, therefore, 
detracts from its value. A moderate percentage of carbonate of 
lime, however, is rather beneficial than disadvantageous, inas- 
much as the carbonic acid which is disengaged from the car- 
bonate when the powdered mineral is treated with acid has a 
tendency to make the superphosphate more porous and bulky, 
and finally to produce a manure in a better mechanical con- 
dition than it can be obtained from minerals altogether destitute 
of carbonate of lime. 
Again, the commercial value of phosphatic minerals is affected 
in a great measure by the percentage of oxide of iron and 
alumina which they contain ; that is to say, the unit per cent, 
of phosphate of lime is worth more in minerals which contain 
