154 
On the Compai'ative Value of 
and in precipitated phosphate of lime. Thus, I showed that 
a given quantity of water dissolves more phosphate of lime from 
fine than from coarse bone-dust ; that more phosphate of lime is 
taken up by a given quantity of water from bone-dust than from 
bone-ash, more from phosphatic guano than from coprolite 
powder, and more from the latter than from apatite and other 
crystalline mineral phosphates. 
Further, that insoluble phosphate of lime, obtained by pre- 
cipitation from its solution, is much more soluble in water than 
phosphate of lime in the shape of the finest bone-meal or bone- 
ash ; and that, recently precipitated, it is very voluminous, and 
in that state about four times as soluble in water as it is after 
it has been dried and heated. 
The greater efficacy of the phosphate of lime as a manure in 
bones in comparison with coprolite powder entirely depends 
upon its finer state of aggregation, and consequently upon its 
greater solubility in the form of bone-dust. 
The general experience of the best farmers during the last 
twenty-five or thirty years has shown most clearly that dissolved 
bone-dust is a more efficacious and more economical manure for 
root-crops than raw bone-dust or even fine bone-meal. As treat- 
ment with acid increases the efficacy of bone-dust, it is evident 
that the same treatment must render more efficacious coprolites 
and other mineral phosphates, which, in the shape of the finest 
powder in which they can be obtained by mechanical means, 
are far less soluble than bone-dust. 
The whole secret of the energetic action of superphosphate 
depends upon the production of most minutely subdivided or 
precipitated insoluble phosphate of lime within the soil itself, 
and not, as is erroneously supposed by some, on the direct 
absorption of soluble phosphates by plants. All soils have the 
power of precipitating, more or less rapidly, soluble phosphate 
from its solution ; but whilst some, like all soils rich in lime, 
effect the precipitation with rapidity, others, deficient in lime or 
other basic elements, only gradually change the soluble into 
insoluble precipitated phosphate of lime. As this change must 
take place before the phosphate can be useful to the growing 
plant, and as even minute quantities of free acid arc injurious to 
the healthy growth of all plants, we can well understand why, on 
soils deficient in lime or other bases, a very acid superphosphate, 
although rich in soluble phosphate, has a less beneficial effect 
upon root-crops than a mineral superphosphate which is poor 
in acid soluble phosphate of lime. By treatment with acid, the 
hard and difficultly soluble phosphates in coprolites and other 
phosphatic minerals are rendered soluble in water in the first 
place, and afterwards obtained as precipitated phosphates in an 
