VII. SPECIAL MANURES 141 



A very simple form of apparatus has been devised by the Badische 

 Anilin und Soda Fabrik, 1 consisting essentially of an iron tube pro- 

 vided at one end with an insulated concentric electrode, from which, 

 on passing the current, an arc springs to the adjacent part of the iron 

 tube which forms the other electrode. A current of air passed through 

 the tube, carries the end of the arc along, so that a column of arc flame 

 is produced, burning quietly in the axis of the tube and surrounded by 

 the air passing through the tube. The gases which leave tbe tube 

 traverse a firebrick flue surrounding the furnace, thus heating the air 

 supply. The solution of calcium nitrate obtained at a later stage can 

 be evaporated by the heat generated by the arc. A large factory to 

 work this process is to be established in Norway, and is expected to 

 use about 140,000 horse-power. 



The commercial nitrate of lime is a white or yellowish substance 

 containing about 13 per cent of nitrogen, readily soluble in water and, 

 unfortunately, deliquescent. In field trials, it has proved quite equal to 

 nitrate of soda, and on many soils, superior, because of its supplying 

 lime as well as nitrogen. 



Ammonium Nitrate. This substance, NH 4 N0 3 , would be a very 

 concentrated nitrogenous manure, containing, as it does, 35 per cent 

 of nitrogen. It has not, unfortunately, been obtained at a sufficiently 

 cheap rate to allow of its being used as a manure. Its deliquescent 

 character would render it difficult to handle even if it could be obtained 

 cheaply. 



Potassium Nitrate. This substance, though doubly valuable as 

 a fertiliser, inasmuch as it supplies both potassium and nitrogen in a 

 directly available form, is too expensive to be used as a manure except 

 under special circumstances. Like sodium nitrate, it occurs as a de- 

 posit in rainless districts in the tropics, especially in India. It was 

 also made by the so-called " Nitre plantations " in France and other 

 countries. These consisted of heaps of earth, old mortar, road scrapings 

 rich in calcium carbonate, etc., mixed with decomposing animal matter, 

 protected from the rain by a shed, and placed on an impervious floor. 

 The heap was watered periodically with urine, liquid manure, or other 

 liquid rich in animal nitrogenous matter. Nitrification under these 

 favourable conditions took place rapidly, and the liquid draining away 

 contained large quantities of nitrates of potassium and calcium. At 

 long intervals the heap was lixiviated with water and the solution 

 mixed with wood ashes or potassium carbonate, when calcium carbonate 

 was precipitated and removed and potassium nitrate was extracted from 

 the solution by evaporation and crystallisation : 



K.CO-3 + Ca(N0 3 ) 2 = CaC0 3 + 2KNO 3 . 



It is probable that the saltpetre of India owes its origin to a similar 



action of nitrification, the potash coming from the minerals in the soil. 



Potassium nitrate is now made in large quantities from sodium 



iBernthsen, Seventh Intern. Conge. Appl. Chem.,1909; Jour. Soc. Chem. Ind., 

 1909, 706. 



