88 PLANT PRODUCTS 



substitute for lime, but it does not contain more than about 

 40 per cent, pure lime and has to compete with lime of 90 

 to 95 per cent, purity in the case of burnt lime. It is not 

 possible to convey it by rail any considerable distance, as 

 the railway freight soon swallows up any advantage of low 

 price. In spite of the fact that the " Chance " mud is a 

 fine precipitate, it runs together in lumps in the soil and is as 

 difficult to distribute as shell lime. Lumps of " Chance " 

 mud can be found in a soil many years after application. 

 Another residue of a similar type is produced from magnesian 

 limestone by the extraction of the magnesia for industrial 

 purposes. The waste is very similar to " Chance " mud, 

 as the amount of magnesia not extracted is very small. 

 Where chalk is obtainable, treating soils with chalk is a well- 

 known process. Even when the soils lie on the top of the 

 chalk, the surface sometimes contains but little lime. Chalk 

 pits are dug in the fields, and the chalk then distributed on 

 the surface. Building mortar can also be employed as a 

 source of lime. The residue of acetylene gas-plants provides 

 a very pure source of calcium hydrate. In a very crude 

 form one may find lime from skin dressers and many small 

 industries. 



The effect of gas lime depends on sulphur and cyanogen far 

 more than upon the amount of lime contained. Fresh gas 

 lime contains considerable quantities of calcium sulphide, 

 which oxidizes on keeping to calcium sulphite. Up to that 

 stage oxidation is rapid, but the further oxidation of calcium 

 sulphite to sulphate in a heap of gas lime is slow, although 

 once it has been distributed in the soil the action is moder- 

 ately rapid. In addition there are sulpho-cyanides, ferro- 

 cyanides, and sometimes cyanides themselves. These are all 

 poisonous bodies, and hence the action of gas lime depends on 

 partial sterilization (p. 90). Gas lime contains about 30 or 40 

 per cent, of calcium carbonate, and when the other substances 

 have had time to decompose, this material produces its 

 effects. A most important lime compound with very different 

 properties is gypsum (hydrated calcium sulphate.) This 

 has no practical resemblance to burnt lime, and its action 



