VII. 



SPECIAL MANUBES 



153 



citric acid solution. In some experiments, the Welters' phosphate was 

 found to be almost as quick in action as superphosphate. 



Another product of somewhat similar character as to citrate- solubility 

 of its phosphates is the so-called Wiborgh phosphate, another German 

 product. It is prepared by heating the crude phosphate (mainly 

 apatite), containing some felspar, with soda-ash, to about 1000 C. 



The reactions are said to be 



3(Ca 3 P 2 O 8 ).CaF 2 + 3Na 2 CO 3 



= 2Na 2 O.10CaO.3P 2 O 5 + 2NaF + SCO, 



K 2 O.Al 2 3 .6Si0 2 + 2Na 2 C0 3 = K 2 O.2Na a O.Al 2 3 .6Si0 2 + 2CO 2 . 



The double sodium-calcium phosphate has a constitution similar to 

 tetracalcium phosphate, and like that substance is soluble in ammonium 

 citrate solution (up to 95 per cent of the total phosphoric acid). The 

 triple silicate of sodium, potassium and aluminium contains its potash 

 in a more readily available form than the original felspar. The com- 

 position of the residue, which is easily powdered, is l : 



Phosphorus pentoxide 



Silica . 



Sulphur trioxide 



Potash . 



Soda 



Lime . 



Magnesia 



Iron oxide and alumina 



Fluorine and loss 



Per cent. 



27-01 



9-99 



0-27 



1-54 



14-69 



38-12 



2-88 



4-50 



1-00 



100-00 



It is probable that a recently devised continuous process of dephos- 

 phorising pig-iron (Talbot's patent) may lead to an increased production 

 of basic slag. 



III. POTASH MANURES. Potash is much more widely distri- 

 buted and less frequently deficient in soils than are nitrogen and 

 phosphoric acid. Potash manures, therefore, are less often used than 

 those already described ; although certain crops, e.g., potatoes, are 

 greatly helped by their application. Formerly, the chief source of 

 potash, both for manurial and other purposes, was the ash left when 

 twigs, leaves, etc., of plants are burnt. Potash occurs in such material 

 as carbonate, and plant ashes are still used to some extent as a manure. 

 They contain the other mineral ingredients, phosphates, lime, etc., but 

 they are especially rich in potash. The chief source of potash is now 

 the immense saline deposits at Stassfurt, and other deposits of a similar 

 kind have been discovered in Thuringia, Brunswick and Mecklenburg. 



These deposits, in some cases hundreds of feet in thickness, rest 

 upon beds of rock-salt, and it was in boring for the rock-salt that they 

 were discovered about 1857. At first they were regarded as useless, 

 as is indicated by the name " Abraumsalzen " (rubbish salts), by which 

 they are still sometimes known. About five or six years later they 



1 Nilson, Jahr. Agric. Chem., 1899, 127. 



