MINERAL PHOSPHATES. 375 



lime, will require five times 1.16 parts, or 5.8 parts, more of 

 sulphuric acid than those which contain none, to render the 

 entire amount of phosphoric acid soluble. Some otherwise 

 very valuable mineral phosphates, as the Sombrero guano, 

 etc., have proved unprofitable for the manufacture of super- 

 phosphates, on account of the frequency of a large admixture 

 of carbonate of lime. 



A more serious feature, however, regarding the application 

 of some mineral phosphates for the latter purpose consists in 

 the fact, that in most cases it becomes impracticable to apply 

 the entire (theoretical) amount of sulphuric acid required to 

 change all the lime, alumina and iron present into sulphates; 

 for the latter render the product not only moist and hygro- 

 scopic, but they are also little qualified for a speedy action on 

 the acid, which tends to leave part of the sulphuric acid, for 

 some time, at least, in a free state. To escape these incon- 

 veniences, less acid is usually used than needed to render the 

 entire amount of phosphoric acid present soluble ; unde- 

 composed bone-phosphates and uncombiued alumina and 

 sesquioxide of iron, are thus common constituents of super- 

 phosphates, which are but recently manufactured from min- 

 eral phosphates ; and as these substances act, in the course of 

 time, quite seriously on the soluble phosphoric acid, by 

 changing it in part into a less valuable form, — so-called 

 "reduced" phosphoric acid, — the cheapest one of two mineral 

 phosphates containing an equal percentage of bone-phosphate, 

 may thus prove to be not always the most economical material 

 to work into superphosphates. 



To derive the full benefit of the comparative cheapness of 

 the mineral phosphates for the manufacture of superphos- 

 phates, requires considerable skill, and some familiarity with 

 chemistry. Large establishments for the manufacture of 

 standard superphosphates, can, for this reason, make no 

 better investment than to put their factory under the super- 

 vision of a good chemist. 



The following tabular statements may convey some idea 

 regarding the price paid for the various bone-phosphates, 

 which enter, at present, into the manufacture of our home- 

 made superphosphates : — 



