56 FERTILIZERS. 



still have a value of from four to six cents per pound, 

 while in the finest ground Carolina rock it would be worth 

 but two cents and a half. The phosphate of lime, as it 

 exists in bones and the mineral rock, is made up of three 

 atoms of lime to one atom of phosphoric acid. In this 

 condition it is insoluble in water, though in bones slowly 

 soluble in the soil; but if two of the parts could be got 

 rid of, then the remainder, made up of one part lime com- 

 bined with phosphoric acid, would be soluble in water. This 

 is accomplished by mixing sulphuric acid and water with 

 finely ground bone, or burnt bone, or finely ground min- 

 eral rock, when the two atoms of lime leave their home in 

 the bone or the mineral, and, combining with the sulphuric 

 acid, become sulphate of lime or plaster, leaving the phos- 

 phoric acid combined with but one atom of lime soluble 

 in water. After having been thus formed, under certain 

 conditions it sometimes takes to itself another atom of 

 lime, and becomes what is called 4 reverted,' or, in other 

 words, has turned back again to an insoluble form. But 

 this two-part lime combination is readily decomposed and 

 rendered soluble when acted on by the carbonic acid pres- 

 ent in the soil, and when so acted on, being in the fine 

 mechanical condition that the action of the acid produced, 

 readily dissolves in water, and becomes plant-food. There 

 are a few terms used, in speaking of phosphate of lime in 

 different forms, that it will be necessary for us to become 

 familiar with. The insoluble combination of three parts 

 lime is often called bone-earth phosphate. When the 

 two parts of lime have been removed, and the phosphoric 

 acid has become soluble, it is often called acid phosphate, 

 but generally superphosphate of lime, the prefix 4 super * 

 being used to indicate that the lime has a proportion of 

 phosphoric acid greater than is natural to it. The term 

 * soluble bone phosphate ' is criticised by Bruckner as a 



