178 THE ANALYSIS OF MANURES. VIII. 



brought about by treatment in alkaline solution with zinc and 

 iron (Mockern's process), or in presence of sulphuric acid by 

 reduced iron (Ulsch's method), or by treatment with a mixture 

 of sulphuric acid, salicylic acid, and sodium thiosulphate 

 (Forster's method). 



In many cases nitrate of soda is valued by "refraction," i.e., 

 by determining the total percentage amount of the impurities 

 present. The substances usually present in Chili saltpetre 

 are moisture, insoluble matter, chlorine, sulphuric acid, mag- 

 nesia, and perchlorate. These constituents are determined by 

 the usual processes. A word or two may be said about the 

 determination of the perchlorate, a substance whose presence 

 and importance in nitrate has only recently been realised. 

 Five grammes of the dried sample are heated for a quarter of 

 an hour over a Bunsen lamp with 7 or 8 grammes of pure 

 calcium hydroxide in a covered crucible. The mass is then 

 transferred to a measuring flask, made up to 128 c.c. with 

 water, digested for an hour and filtered (3 c.c. is taken as the 

 volume of the undissolved matter, so that 125 c.c. of solution 

 are really taken). To 100 c.c. of the nitrate (= 4 grammes of 

 the sample) dilute nitric acid is added until exactly neutral and 

 the chloride present is then determined by titration with silver 

 nitrate. After deducting the chlorine present, originally as 

 chloride (determined by direct titration without treatment with 

 lime), the chlorine found is calculated to perchlorate." 



In the case of most manures the determinations of the 

 various constituents are made by the usual quantitative 

 methods, but in some cases shorter and easier, though perhaps 

 less accurate, methods are employed. A few of these rapid 

 methods may be mentioned. 



Phosphoric Acid. In the presence of aluminium, iron, and 

 calcium in ordinary analytical work it is generally considered 

 necessary to first precipitate the phosphoric acid with ammo- 

 nium niolybdate, wash the yellow precipitate until the above- 

 mentioned metals are removed, redissolve the precipitate in 

 ammonia and precipitate with magnesia mixture. Instead of 



* Blattner and Brasseur, Chem. Zeitung 1900, 767. 



