LIMESTONE 161 



In the " Georgical Essays " (1777 edition), we find an article by 

 T. Henry, F.R.S., on the " Action of Lime and Marl as Manures," 

 in which the following statements occur: 



"The lime, that we may come nearer to nature in our imitation, should not 

 only be slacked, but be exposed to the open air, and often turned for several 

 months, that it may recover its air; for it requires a long series of time be- 

 fore it recovers the whole of which it has been deprived in calcination. . . . 



"I find that Doctor Home thinks that lime produces little effect on vegeta- 

 tion till it is become effete. It may be known to have recovered its air by its no 

 longer forming lime water, and by effervescing violently with acids without 

 growing hot. If, however, the method described in the last note be used, it 

 will be sufficient, if the lime be fallen, without waiting for the recovery of its 

 air, as this point will be acquired during the long time which the mixture is to 

 be exposed to the action of the atmosphere. . . . 



"Upon the whole, may we not conclude that lime, in most cases, is a stronger 

 manure, when it has recovered the air of which it has been deprived in calci- 

 nation, than it is when brought fresh from the kiln ; and that when procured for 

 the purposes of agriculture, its efficacy and permanency will in general be 

 increased, by mixing it, in its effete state, with the other ingredients which enter 

 into the composition of marl?" 



When limestone is burned, the calcium carbonate (CaCO 3 ) is 

 decomposed, the carbon dioxid (CO 2 ) passes off as a gas, leaving 

 the product calcium oxid (CaO), which constitutes 56 per cent by 

 weight of the limestone used. 



When exposed to the moisture of the air or soil, the quicklime 

 (CaO) quickly takes up water and forms calcium hydroxid, 

 Ca(OH) 2 , sometimes called hydrated lime, which means merely 

 water-slacked lime. The product is the same whether the slack- 

 ing (hydrating) is performed by the manufacturer at large expense, 

 or by the farmer at little or no expense. 



When slacked lime is exposed in the air or soil, carbon dioxid is 

 gradually absorbed, and the calcium carbonate is thus reformed. 

 Thoroughly air-slacked lime is exactly the same material as fine- 

 ground limestone. In other words, no matter what form of lime 

 we apply to the soil, the benefit derived during the subsequent 

 months or years is due to one and the same compound, calcium 

 carbonate. 



These facts alone would be sufficient, perhaps, to lead one to use 

 ground natural limestone in preference to the disagreeable caustic 



