MANURES AND THEIR USES. 
213 
treated thus, they were gradually employed in smaller and 
smaller pieces, until at last they are reduced to actual powder 
before using, and it is found that the finer the bones are ground, 
the more quickly is the improvement of the soil evident. 
About two-thirds of the weight of bones consist of mineral 
matter, which is composed chiefly of phosphate of lime, and 
their value as a fertiliser depends to a very great extent on their 
phosphoric acid, which, in combination with the lime, forms 
phosphate of lime. The organic portion of bones (cartilage, 
gelatine, &c.) contains nitrogen, which, decomposing in the soil, 
forms ammonia, and assists in the fertilising action of the 
phosphate of lime. 
One of the best forms of nitrogen for manure is that furnished 
by the organic matter of bones, after having undergone a partial 
decomposition by acid ; and it is to this fact, added to the 
superiority of the phosphate associated with it, that the pre- 
ference given by gardeners to bone manures when of good 
quality is to be attributed. Several kinds of bone-meal are now 
to be met with, and are generally considered superior to ordinary 
bone-dust, since many of them contain dried animal matter in 
addition to the bone. 
Superphosphate of Lime. 
To bring bones into a condition whereby their constituents 
become more quickly available for plant food would naturally 
attract the attention of gardeners and farmers. When bones 
are ground to a powder, wetted, and then treated with sulphuric 
acid, disagreeable gases pass off, and most of the bones dissolve. 
In time the residue dries more or less into a solid mass, and if 
the quantity of acid and bones have been properly proportioned 
a moist greyish substance has been formed, which is known as 
superphosphate of lime. 
Eaw bones are not much used for making superphosphate, 
but it is now almost wholly prepared by mixing sulphuric acid, 
with bone-ash (which is imported in considerable quantities from 
South America) and various mineral phosphates, such as 
coprolites and apatite. 
The solubility of phosphate of lime depends on what kind of 
phosphate it is, whether it is made from bones, coprolites, or 
other mineral phosphates. The insoluble is called '* tribasic," 
