BONE-DUST AND SUPEKPHOSPHATE OF LIME. 317 



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 phos- 

 phate 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 * dis- 

 solving ' 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 ques- 

 tion has been fully decided by the most satisfactory experiments." 



"Before you and the Deacon discuss that subject," said the Doc- 

 tor, " 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 phos- 

 phoric 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 phos- 

 phoric 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 



