On Superphosphate of Lime. 



207 



Sit present : these are the neutral phosphate of lime, sometimes 

 called bone earth phosphate," because it forms the chief earthy 

 constituents of bones; and the bi-phosphate, or, as it is otherwise 

 named, the acid phosphate of lime. 



The term bi-phosphate of lime will, in the present paper, 

 always be employed in speaking of the true chemical substance, 

 the term " superphosphate " being reserved for the commercial 

 mixture sold under that name. It is to be borne in mind then 

 that, in the remarks that immediately follow, we are speaking only 

 of the true chemical compounds ; the phosphate and bi-phosjihate 

 of lime, as we are practically acquainted with them, being always 

 largely mixed with other substances, as will presently appear. 



Pure neutral phosphate of lime consists in the 100 parts of — 



Phosphoric Acid .... 48^ 

 Lime 61^ 



100 



It is only very slightly soluble in water, but more so in water 

 in which carbonic acid gas is dissolved, and also, it is said, in 

 solution of common salt and some other salts. The ease with 

 which any substance can dissolve in water or other solvents is 

 very much influenced by its mechanical condition : and this for 

 obvious reasons. The more finely we powder and break up a 

 lump of rock-salt or of alum, the more readily will it dissolve in 

 water. There is a greater amount of surface exposed to the 

 solvent action of the water, and more effect produced in a given 

 time. Sooner or later, however, the rock salt and the alum will 

 dissolve whatever may be the size of the lumps. But in some 

 substances known to chemists an alteration in the state of aggre- 

 gation — in the subdivision of the particles — gives rise to altogether 

 new and different chemical properties, at all events in reference 

 to solubility. Phosphate of lime itself is one of these. 



Phosphate of lime, as we shall presently have to explain more 

 in detail, occurs in a mineral state, and one variety of this 

 mineral phosphate is known to us in the form of the coprolites 

 of the crag and green sand formations. Some of these coprolites 

 are excessively hard, so much so as to take a good polish. We 

 have no reason to believe that they have been subject to the 

 action of heat, but have become solid as we see them by rolling 

 over and over, probably under considerable pressure. By 

 proper means these stones can be reduced to a very fine powder, 

 so as to be entirely free from gritty feeling when rubbed between 

 the fingers. Now any attempts to dissolve this coprolite powder 

 in acetic acid (vinegar) would entirely fail : some small portion 

 of the phosphate might, indeed, be, after a long time, found in 

 solution, but the action at the best would be very inconsiderable. 



