availability of bone-dust is by mixing
it with sulphuric acid.

The phosphate of lime in bones is insoluble in water, though rain water
containing carbonic acid, and the water in soils, slowly dissolve it. By
treating the bones with sulphuric acid, the phosphate of lime is
decomposed and rendered soluble. Consequently, bone-dust treated with
sulphuric acid will act much more rapidly than ordinary bone-dust. The
sulphuric acid does not make it any _richer_ in phosphoric acid or
nitrogen. It simply renders them more available.

"And yet," said the Doctor, "the use of sulphuric acid for 'dissolving'
bones, or rather phosphate of lime, introduced a new era in agriculture.
It is the grand agricultural fact of the nineteenth century."

"It is perhaps not necessary," said I, "to give any direction for
treating bones with sulphuric acid. We have got beyond that. We can now
buy superphosphate cheaper than we can make it from bones."

"But is it as good?" asked the Deacon.

"Soluble phosphate of lime," said I, "is soluble phosphate of lime, and
it makes no difference whether it is made from burnt bones, or from
phosphatic guano, or mineral phosphate. That question has been fully
decided by the most satisfactory experiments."

"Before you and the Deacon discuss that subject," said the Doctor, "it
would be well to tell Charley what superphosphate is."

"I wish you would tell me," said Charley.

"Well," said the Doctor, "phosphate of lime, as it exists in bones, is
composed of three atoms of lime and one atom of phosphoric acid.
Chemists call it the tricalcic phosphate. It is also called the basic
phosphate of lime, and not unfrequently the 'bone-earth phosphate.' It
is the ordinary or common form of phosphate of lime, as it exists in
animals, and plants, and in the various forms of mineral phosphates.

"Then there is another phosphate of lime, called the dicalcic phosphate,
or neutral phosphate of lime, or reverted phosphate of lime. It is
composed of one atom of water, two atoms of lime, and one atom of
phosphoric acid.

"Then we have what we call superphosphate, or acid phosphate of lime, or
more properly monocalcic phosphate. It is composed of two atoms of
water, one atom of lime, and one atom of phosphoric acid. This acid
phosphate of lime _is soluble in water_.

"The manufacture of superphosphate of lime is based on these facts. The
_one-lime_ phosphate is soluble, the _three-lime_ phosphate is
insoluble. To conv