“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 convert the latter into the former, all we have to do is to take away two atoms of lime.