THE MANUFACTURE OF FERTILIZERS 231 



charge, and the more rapidly will the particles travel. In 

 practice the necessary charge is produced by connecting a 

 source of alternating current to the low-tension terminals of 

 a transformer, the high-tension terminals of which are 

 connected to a mechanical rectifier, which produces a direct 

 current. The receiving electrode connected with the 

 precipitating plate is usually earthed, for convenience and 

 safety. The dust collected on the plate by these means is 

 knocked off by a tapping appliance and transferred to a 

 spiral conveyor, by which it is carried to the store. Much 

 of the dust of high fertilizing value which is produced in 

 grinding machinery may be recovered in this way, as well 

 as many other special flue dusts, for which this type of appa- 

 ratus has been mostly used. 



When the wires used in the Cottrell plant are covered with 

 cotton or asbestos, the long fibres act as discharge points, 

 which, owing to their fineness, are superior to metallic points. 

 The Anaconda Copper Mining Company, Montana, U.S.A., 

 use over 100 miles of chains suspended between plates. 

 When the plates are thickly coated the current is cut off* 

 and the dust falls into a hopper. 



The Cottrell process has also been used to precipitate tar 

 found in coal gas, preparatory to directly fixing the ammonia. 



A further type of improvement which may be looked for 

 is the better utilization of crude potash salts, which would 

 be much better converted into sulphates than left as chlorides 

 (p. 170). The ordinary salt cake furnace will suffice for 

 such a purpose, and instead of wasting much of the sodium 

 and magnesium salts mixed with the potash, these could 

 be turned into the corresponding sulphates and exert their 

 fertilizing value. Up to the present, little practical use has 

 been made of sodium and magnesium sulphates, although 

 Rothamsted has been showing for about 70 years that these 

 materials are by no means to be despised as fertilizers. 



The war has left Great Britain possessing much new plant 

 and many novel industries. At the time when hostilities 

 ceased, this country was manufacturing about 100,000 tons 

 per annum of nitric acid and sulphur tri oxide, with an efficiency 



