VII. SPECIAL MANURES 159 



lime containing much magnesia may retain its caustic condition 

 (because of the MgH 2 2 ) in the soil for a much longer time than a 

 pure lime would under the same conditions. 



Too heavy or too frequent dressings with lime produce harm by 

 exhausting the stores of nitrogenous matters contained in the humus 

 of the soil. 



The very valuable flocculating effects of lime upon clay have 

 already been discussed. 1 



Ferrous Sulphate, FeSO 4 .7H 2 O, known commercially as " cop- 

 peras " and " green vitriol," is not often used as a manure. Iron is 

 usually sufficiently abundant in a soil. Griffiths 2 greatly extols the 

 application of small quantities (up to ^ cwt. per acre) of ferrous 

 sulphate as a top-dressing for meadows, and also for beans, cabbages, 

 potatoes, mangolds and cereals. He found that the iron sulphate in- 

 creased the amount of chlorophyll in the green portions of the plant, 

 that the crops were larger, richer in solid matter, albuminoids and 

 phosphates, and that the growth of mosses, etc., was hindered or pre- 

 vented. He also claims that the iron, to a certain extent, performs 

 the functions of potash in the plant, and that in many cases ferrous 

 sulphate may advantageously be substituted for kainite or other potash 

 manures. Continental experimenters have confirmed in many in- 

 stances the claims of ferrous sulphate as a manure. In addition to its 

 specific action, it, like other sulphates, probably supplies sulphur. 



Gas Lime. In the preparation of coal-gas, various sulphur com- 

 pounds are expelled by distillation from the coal, and though a 

 considerable proportion of these are collected in the ammonia-liquor 

 and tar, some find their way into the gas, and inasmuch as they are 

 highly objectionable there, purification has to be resorted to. One of 

 the common methods of removing sulphuretted hydrogen and carbon 

 disulphide from the coal gas is to pass it into chambers containing 

 layers of slaked lime, when the following reactions occur : 



CaH 2 O 2 + H 2 S = CaS + 2H,O 

 CaS + S 2 = CaCS 3 . 



Simultaneously the carbon dioxide is also removed 

 CaH 2 2 + C0 2 = CaC0 3 + H 2 0. 



Small quantities of cyanogen are also absorbed. 



The spent lime is a complex mixture calcium sulphide, sulpho- 

 carbonate, sulphite, carbonate, thiocyanate and hydrate being the chief 

 ingredients, with varying small quantities of ammonia, cyanides, fer- 

 rocyanides, etc. Many of these compounds are powerful plant poisons, 

 and fresh gas lime is extremely destructive to all plant life. On exposure 

 to air and rain, especially if mixed with soil, absorption of oxygen takes 

 place, and the sulphides, sulphites, etc., are oxidised first into thiosul- 

 phate and finally into sulphate : 



1 Vide p. 45. 2 Jour. Chem. Soo., 1885, Trans., 54 ; 1886, Trans., 121. 



