On Sujjerplwsphate of Lime. 



229 



is of much more value for soap and candle making than it can be 

 as manure. Bones so treated will of course come cheaper to the 

 manure-maker after a valuable material has already been 

 extracted from them : the same mav be said of the gelatine or 

 glue. As a whole there will be a greater gain to the community 

 by the separation of the fat, and more or less of the glue, than 

 by the employment of these substances as manure. It will be 

 for the maker of superphosphate to decide whether he can better 

 buy the bones in this state or carry on the preliminary operations 

 himself. If he limits himself to the former he will do well to 

 ascertain that the bones are not over wet or mixed with such a 

 quantity of impurity as to diminish materially their value. 



If a mineral phosphate is to form the basis of the manure, he 

 should be careful that, with a fair proportion of phosphate, it 

 does not contain too large a percentage of carbonate. As we 

 have already shown, every pound of carbonate of lime leads to a 

 certain waste — an equal waste — of acid ; the same is true of 

 fluoride of calcium, which equally requires to be overcome 

 before the phosphate can be acted upon. Care must also be 

 taken that the phosphoric substances, particularly of the chalk 

 and greensand formation, are really what they profess to be. 

 With a similar shape and an appearance which would deceive any 

 but those who are accustomed to them, some of the fossils of the 

 greensand are found to be almost entirely composed of carbonate 

 of lime or of flint ; and even where the genuine substances are 

 collected they are frequently mixed up with so much sand and 

 worthless matter as to render them wholly unfit for the manu- 

 facturer's purpose. The same caution is required in the pur- 

 chase of bone-ash and animal charcoal ; I have known several 

 instances where a substance has been bought, and even used, for 

 a considerable period as animal charcoal, which upon examina- 

 tion was found to contain no portion of phosphate of lime. 



Of the acid I have already spoken : if oil of vitriol is employed, 

 its density and consequent strength should be ascertained ; it is, 

 however, much more economical to employ the weaker or brown 

 acid. Next to the purity of the materials themselves is their 

 mechanical condition. Whether bones or mineral phosphate are 

 employed, they should be reduced to the finest state practicable ; 

 the finer the better. Contact of the acid and phosphate in every 

 part is essential to perfect action, but the very nature of the 

 products makes it difficult of accomplishment. The sulphate of 

 lime formed, being insoluble, clogs up the entrance to the bones 

 and covers over the particles of the mineral phosphates so as to 

 prevent the remainder of the acid from being brought into play. 

 It is by no means unusual to find a specimen of superphosphate 

 in which abundance of acid has been employed, but in which 



