174 THE ANALYSIS OF MANUKES. VIII. 



in solution of ammonium citrate), and dicalcium phosphate, 

 Ca 2 H 2 P 2 8 ( = CaHP0 4 ), together with the phosphoric acid in 

 combination with iron and aluminium (soluble in solution of 

 ammonium citrate), all expressed in terms of tricalcium phos- 

 phate. The phosphoric acid soluble in ammonium citrate 

 solution is known as "reverted," "retrograde," or "reduced." 



The " hydrated sulphate of lime" obviously refers to the 

 compound analogous to gypsum, CaS0.2H 2 0. 



" Organic matter and water" requires no explanation ; it is 

 loss on ignition after deducting the water contained in the 

 " hydrated sulphate of lime," which would also be expelled; 

 why this allowance is made for mere water of crystallization 

 in the case of calcium sulphate and no allowance for the 

 necessary water of constitution of the mono-calcium tetra- 

 hydrogen phosphate, CaH 4 P 2 O 8 , it is difficult to understand. 



" Alkaline salts" is another unsatisfactory item ; it is hard 

 to say exactly what it means. 



" Silica" usually represents the matter insoluble in acids 

 and often consists mainly of real silica, Si0 2 , though it may 

 contain other mineral fragments. 



A much more scientific and in every way more satisfactory 

 way of reporting such an analysis, adopted in more recent 

 work, is as follows : 



% 



Total phosphorus pentoxide ... 



fSoluble 



' ' Reverted , " " reduced , " " retrograde , ' ' 

 of which is-] 



or, better, "citrate soluble 



^Insoluble ... 

 Sulphur trioxide 

 Loss on ignition 

 Potash (if any) 



In the case of nitrogenous manures it is often the practice 

 to give the nitrogen as equal to % of ammonia ; if the per- 

 centage of nitrogen is given as well, this plan is perhaps 

 permissible, but it would be much more intelligible to give the 

 latter than the former. The percentage of ammonia equiva- 

 lent to the nitrogen present should therefore not be given 



