240 CHEMICAL FERTILIZERS 



can be made by taking a saturated solution of mercuric 

 chloride at 60 F. and adding 5 % of hydrochloric acid. 

 Such a solution can be diluted with from ten to a hundred 

 times its bulk of water, according to need. As a precaution 

 against blunders it is advisable to colour the stock solution 

 with a little indigo sulphonate or aniline blue. Mercuric 

 chloride, in the presence of air and hydrochloric acid, slowly 

 oxidizes many organic dyes, but indigo stands well. 



Copper. The copper compounds used in agriculture 

 are made from copper sulphate. In the manufacture of 

 copper sulphate, copper containing iron and sulphur is 

 roasted with the addition of more sulphur if necessary, 

 dropped into sulphuric acid diluted with water and the solu- 

 tion cooled till crystallization takes place. By adjusting 

 temperatures it is possible to reduce the amount of iron 

 entering into solution. In any case, some iron separates 

 out with the copper in the form of CuSO 4 .3FeSO 4 .28H 2 O. 

 This compound decomposes at about 180 C. (350 F.), 

 when ferrous sulphate free from copper crystallizes out. 

 High pressure steam boilers permit this operation to be 

 carried out. Even at atmospheric pressure, recrystalliza- 

 tion of copper sulphate containing ferrous sulphate yields 

 a crop of crystals containing much less iron. Considerable 

 quantities of crude copper sulphate containing much ferrous 

 sulphate is placed on the market. For some purposes these 

 crude mixtures can be used in agriculture, but the general 

 experience is against them, since the average cost of the copper 

 contained exceeds that of the same amount of copper in 

 good quality copper sulphate. 



For most agricultural purposes, the physical condition 

 of the sulphate of copper sulphate is very important. I,arge 

 crystals are troublesome when making solutions on the farm. 

 This difficulty can be removed by grinding, but the best 

 method of dealing with the manufacture of copper sulphate 

 for agricultural purposes is to crystallize quickly with agita- 

 tion. For this purpose, a hot solution of copper sulphate 

 containing \ % free sulphuric acid is run into a sheet copper 

 vessel, surrounded with water. Agitation is best obtained 



