18 ON THE USE OF LIME, MARL, AND SHELL-SAND. 



lime to fine powder is spontaneous slaking in covered heaps. 

 The lime is laid up in heaps, and covered with grass sods, earth, 

 or the clearings of ditches, and then left for three or four weeks, 

 or until it has completely fallen to powder. Covering with sods 

 and earth excludes to a great extent heavy showers or long-con- 

 tinued rain, whereby the quicklime is apt, at least in part, to 

 run into mortar. Should this take place, notwithstanding the 

 precaution of keeping the heaps covered with earth or grass sods, 

 it is advisable to mix the mortary portion of the heap with the 

 rest, or, better still, with some fresh quicklime, before distributing 

 it over the field. 



Some kinds of lime, especially inferior limes, do not slake 

 spontaneously, and require to be watered in order to fall to powder. 

 Slaked in either way, lime greatly increases in bulk the more 

 the better it is. Good limes generally increase 3 to 3J times, 

 and seldom as much as 4 times ; whilst bad limes, particularly 

 those rich in earthy matter, increase rarely more than twice in 

 bulk, and often less. 



Slaked lime contains water in an invisible form, or chemically 

 combined water, and is called by chemists hydrate of lime. 

 JOO parts of lime-hydrate, or dry slaked lime, when pure, 

 contain 



Lime 76 



Water 24 



Quicklime differs materially in its physical and chemical pro- 

 perties from the stone from which it is obtained. Whilst lime- 

 stone is hard and dense, and nearly insoluble in pure water, 

 quicklime falls readily to a fine powder, which is soluble in 

 about 770 parts of water. Quicklime has a hot, alkaline taste, 

 and causes, amongst other effects, the resolution of vegetable and 

 animal matters into simple, volatile, or soluble combinations. 

 The changes limestones undergo in burning are thus partly of 

 a mechanical, partly of a chemical kind. The mechanical 

 changes enable the farmer to reduce the lime by slaking into an 

 extremely fine powder, which can be readily and thinly spread 

 upon, or intimately mixed with, the soil. The chemical change 

 consists chiefly in the expulsion of carbonic acid, and the con- 

 version of the mild into caustic lime. 



Slaked lime, when exposed for some time to the atmosphere, 

 gradually absorbs carbonic acid, and returns in part to a mild 

 condition, or becomes partly carbonate of lime again. Still, 

 however long lime may have been exposed to the influence of 

 the atmosphere, it never becomes entirely reconverted into car- 

 bonate. 



With a view of ascertaining to what extent slaked lirne, which 

 had been exposed for some months to the atmosphere, had 



