230 SOILS AND MANURES 



but is of great practical importance because it accounts, 

 in large measure, for the peculiar properties upon which 

 the manurial value of slag depends. Basic salts are 

 potentially bases, and exhibit the characteristic properties 

 of the class, of which the most important is a capacity 

 to react with acids. They are therefore generally much 

 more active than the corresponding normal salts. This is 

 conspicuously the case in regard to basic phosphate of 

 lime. It is practically insoluble in pure water, but is 

 attacked by water containing carbonic acid in solution. 

 It is soluble to a very large extent in neutral citrate of 

 ammonia, and in dilute citric acid solutions. The native 

 (tricalcic) phosphates are practically insoluble in these 

 reagents. The manurial effects of basic slag seem to 

 show that the phosphates are more readily available to 

 the plants than those of the native phosphates. This 

 is probably due to the greater solubility which apparently 

 depends upon the basic character of the phosphates. 



It would be a mistake to suppose that basic slags are 

 so-called because they contain basic phosphates. The 

 name was originally applied, not to Thomas' phosphate 

 at all, but to the raw substance, in contradistinction to 

 the acid slags which are also employed in iron-works, 

 The latter generally consist of silica, and are used 

 chiefly as a flux for basic impurities. They contain 

 no phosphates, and are of no value for manurial pur- 

 poses. Thomas' phosphate, it will be seen therefore, 



structure of the tricalcic and tetracalcic phosphate may be indicated 

 as follows : 



O O 



Tricalcic phosphate. 



O O 



-O-Ca-O-Ca-O 



Tetracalcic phosphate. 



