134 CHEMICAL MANURES. 



broken, as is the case when orders are being dispatched. When 

 superphosphates of this kind arrive at their destination they some- 

 times yield, on analysis, different results from those obtained on 

 dispatch, but the sample then retained shows identical results, 

 which shows that the superphosphate retrogrades in sample bottles. 

 If retrogradation cannot be stopped, nor the chemical action which 

 determines it, one can in a way prevent it in a bad superphosphate. 

 It suffices to dry it as completely as possible, store it cold, shorten 

 the storage, or preserve the superphosphate in a thin layer in the 

 beginning by spreading it over all the available space, extracting the 

 lumps by sifting, using it to prepare superphosphate of ammonia 

 (the sulphate of ammonia may hinder retrogradation) or low 

 strength superphosphates. By mixing 10 per cent of wood sawdust 

 of bad quality with an 18 per cent P^O- superphosphate which had 

 retrograded 1-5 per cent, it was observed that the phosphoric acid 

 did not retrograde further in the low strength manure thus prepared. 

 In this case the amount of liquid contained in the superphosphate 

 HgPO^ + H^O lost 45 per cent of its mobility in consequence of 

 this dilution. On the other hand, the superphosphate was loosened 

 from its pressure, the latter being spread over all the ballast. It 

 is clear that this remedy has limits, but it gives good results. 



Forestalling Retrogradation bij ScJiitchfs Method. — To ascertain 

 if a superphosphate is going to retrograde, and at what moment it 

 will retrograde, are points which it is important for the manure 

 manufacturer to know. Schucht has used for years a method which 

 gives results appreciably agreeing with facts. It is generally very 

 difficult to get access to the lower layers of a superphosphate heap. 

 It is, therefore, impossible to analyse them, and another means of 

 control must be found. This means is supplied by the retrograda- 

 tion indicator about to be described. The apparatus consists of a 

 graduated lever a, on which is a movable runner of 15 kg. (33 lb.). 

 This lever is movable, round a point of support b, fixed on a fork c. 

 A very strong flat-bottomed porcelain vessel e, is used to receive the 

 superphosphate, which is compressed by means of the plug / fixed 

 on the lever. The operation is conducted as follows : 100 grms. of 

 superphosphate are run into the porcelain dish with 25 grms. of 

 hot water, and the mixture crushed with the pestle ; the excess of 

 water is expelled on the water bath and the vessel is put under the 

 press plug, which compresses the contents of the dish. The ap- 

 paratus is kept under pressure for twenty-four hours at a temperature 

 of 50° to 70° C. (122° to 158° F.), water, soluble phosphoric acid, and 

 free phosphoric acid are estimated in the superphosphate, treated as 

 above, after having already made the same estimations in the 

 original superphosphate. By comparing the results of the two 

 analyses a valuation of the superphosphate can be made. The 

 differences existing between manufacturing localities and in storing 



