80 CHEMICAL MANUEES. 



sulphuric acid. There is then only obtained a wet tacky mass, the 

 acid uniting with the gypsum to form CaH2(S0J-. It is only after 

 a certain time, after numerous treatments followed by drying, that 

 this free acid ends by acting on the non-decomposed phosphate, but 

 not on the phosphate of iron. On the other hand, the hydrated 

 phosphate of iron is soluble in the free phosphoric acid of the 

 superphosphate, but only in a transitory fashion. 



Oxide of alumina in the proportion in which it generally occurs 

 as AlPO^ in raw phosphates does not exert directly any influence 

 on the retrogradation of the phosphoric acid. Contrary to what 

 takes place with oxide of iron, the hydrated oxide of alumina 

 AloO^H.^O does not become soluble in acid until after ignition. 

 The hydrated phosphate of alumina as well as the anhydrous 

 phosphate of alumina is soluble in the precipitated condition in 

 phosphoric acid. That is why it is unnecessary to eliminate the 

 phosphate of alumina from raw phosphate so long as it exists in 

 normal limits. Alumina as silicate may prove injurious ; if it be 

 not decomposed by the sulphuric acid it may in certain cases cause 

 the retrogradation of a portion of the phosphoric acid. 



Organic Matter. — According to the origin of the phosphates 

 matter is present which is carbonized in the mixing process 

 •either by sulphuric or phosphoric acid. The dust given off by 

 •certain phosphates (Algeria, Gafsa) during grinding diffuses a bad 

 smell which it is impossible to eliminate during manufacture. 

 Ignition alone suppresses it. Coprolites, Carolina phosphate, pebbles, 

 Algerian phosphate give off an odour of ichthyocol or of naphthene 

 analogous to that given off on heating petroligneous shale, limestone, 

 etc. These odours are due to the decomposition of the remains of 

 the fat of marine animals. More often, however, a penetrating odour 

 is given off from phosphates, resulting from the decomposition of 

 the remains of slightly volatile organic matter, which are 

 not decomposed by sulphuric acid except at the high tem- 

 perature incidental to the mixing process. These remains 

 form a nitrogenized and sulphurous charcoal, which is then con- 

 verted into compounds with an acrid and penetrating odour. 

 Certain coprolites even exude a sort of tar during drying, forming 

 up to 0'5 per cent of the mass. The above shows how variable the 

 raw materials are which are met with in course of manufacture. 



Method of Determining the Amount of Acid to use in Order to 

 Bender the Phosphate Soluble. — It was a long time before an agree- 

 ment could be come to as to the quantity of sulphuric acid to use 

 to render the raw phosphate soluble. There v/as a plausible 

 reason, for the manufacturer had the alternative of completely dis- 

 solving the phosphate by the use of a large quantity of sulphuric acid, 

 and then the superphosphate so produced was too moist to be 

 spread by means of the drill, or of using less acid and only incom- 



