CHEMISTEY OP SOILS 119 



But a further change soon takes place; the calcium 

 hydrate, or slaked lime, combines with carbon dioxide in 

 the air and re-forms calcium carbonate, thus 



ga(OH) 2 + C0 2 CaC0 3 + H 2 



Slaked lime. Carbon dioxide. Calcium carbonate. Water. 



The net result of all these changes is, therefore, to repro- 

 duce the same substance with which they began. All that 

 is accomplished is to obtain the substance, calcium car- 

 bonate, in fine state of division suitable for admixture with 

 the soil. 



Within the last few years a substance called ground lime 

 has been put on the market and has been somewhat exten- 

 sively employed. This is simply quicklime mechanically 

 ground to a fine powder. It is spread on the land in that 

 form and there becomes first slaked and then carbonated 

 as above described. It is said to be more effective than the 

 ordinary lime, and consequently a smaller quantity wil] 

 suffice for a given purpose. In cases where difficulties of 

 transport are encountered this may be an important con- 

 sideration, but it is more expensive to purchase and 

 requires very careful handling. If allowed to become damp 

 the lime slakes, the bags burst, and damage may be done. 

 When first applied it probably acts more vigorously on the 

 organic matter in the soil, but this effect must be transient, 

 and its chief advantage over ordinary lime is probably 

 due to the more thorough distribution of the lime through 

 the soil when it is applied in this form. 



Lime is used in gasworks to absorb the sulphuretted 

 hydrogen and carbon dioxide from the gas produced by 

 destructive distillation of coal. The spent product, called 

 " gas lime,'.' is sometimes used for agricultural purposes. 

 It contains from 10 to 20 per cent, of calcium hydrate, from 

 20 to 25 per cent, of carbonate, varying quantities of 

 sulphide, sulphite, thiosulphate, and other impurities. The 



