244 CHEMICAL FERTILIZERS 



the dodecahydrate is also obtained. A little over ij times 

 its weight of lead acetate is added to produce a paste of lead 

 arsenate. Without a knowledge of the exact composition 

 of the ingredients, a safe paste is hard to produce. A pure 

 heptahydrate of sodium arsenate contains 59-6 % of anhydrous 

 sodium arsenate, whilst the pure dodecahydrate contains 

 46*27 %. It takes 3 '056 parts of pure crystallized lead 

 acetate to precipitate i part of dry sodium arsenate. 



L,ead arsenate can also be made more directly. Four 

 parts of arsenious acid are added to 3 parts of nitric acid of 

 i -35 specific gravity. On the large scale heat is evolved 

 and much nitric acid is given off, much of which can be con- 

 densed in a tower full of wet coke. On the small scale 

 external heat and more acid may be needed. In either 

 case a hot syrupy liquid is left consisting of arsenic acid 

 containing traces of arsenious acid and some nitric acid. 

 The nitric acid must be reduced by heat till it does not exceed 

 i % of the mass. Fourteen parts of litharge are now added 

 with 10 parts of water, when slight heat is evolved. Warming 

 the mass hastens the combination till only lead arsenate is 

 left with a little lead nitrate. If the amount of nitric acid 

 was too great a little sodium carbonate must be added, 

 till neutral to litmus. If the amount of nitric acid left in 

 the arsenic acid was too small the reaction becomes very 

 slow. The resulting paste will show at once if the right 

 quantity of water was added. The compound actually used 

 is Pb 3 (AsO 4 ) 2 mixed with water. 



Ferrous Sulphate. This material in the crystallized 

 form, FeSO 4 . 7H 2 O, is produced as a by-product in the 

 separation of copper in the wet way, by throwing iron scrap 

 into weak solutions of copper sulphate. The ferrous sulphate 

 can be recovered by crystallization. Tin plate works also 

 contribute to the supply. 



Ferrous sulphate can be used in sprays in the same way 

 and for the same purposes as copper sulphate. It is, however, 

 much less active, and almost saturated solutions must be used. 

 The addition of free sulphuric acid increases the germicidal 

 effect. The spray from ferrous sulphate is ruinous to clothing. 



