140 INSECTICIDES, FrXGICIDES, AND WEED KILLEES. 



Preparation. — Quicklime may be made from cail)onate of lime, 

 either in the pure state as marble or chalk, or impure as marl or lime- 

 stone. Limestone is bm-nt in lime kilns by heating it gradually to a 

 red heat. This heat, kept up for three days on an average, decom- 

 poses the limestone into quicklime and carbonic acid. 



CaCOy = CaO + CO.,. 



As soon as the burning is finished and the mass cooled, it is packed 

 in casks, and hermetically sealed so that the air does not affect it by 

 its carbonic acid and moistm'e. 



Properties, — Lime is converted by the action of water into hydrate 

 of lime (CaOH^) or slaked lime. In slaking, the lime gives off much 

 heat and swells or increases in bulk. In contact with carbonic acid 

 calcium hydrate is converted into calcium carbonate (CaCOg). Milk 

 of lime exposed to the air does not keep more than two to three 

 months, and gradually loses its caustic properties. Lime is very 

 slightly soluble in water, which only dissolves at the ordinary tempera- 

 ture 0-14 percent; hot water only dissolves O'l percent. Lime stirred 

 up with water remains suspended, and forms a milky liquid, " milk of 

 lime ". This milk is the fatter the more pure the lime from which it 

 is made. There are two sorts of lime, fat lime and thin lime. The 

 first comes from the burning of almost pure limestone, such as chalk 

 or marble ; it is white, and gives off much heat on slaking and increases 

 two to three times in bulk ; it forms with water a fatty and binding 

 paste. Thin lime, on the other hand, is produced by the impure lime- 

 stones referred to above ; it is grey, and disengages little heat on slaking ; 

 mixed with water it hardly swells and forms a short paste. For 

 agricultural purposes, and particularly for agricultural medicine, it is 

 necessary to choose a fat lime, which yields on slaking a very caustic 

 impalpable powder, which, stirred up with water, yields a milk with 

 great adherence. To increase this still more, a little cement, bullocks' 

 blood, potters' clay, or cow dung is added. The white colour, often too 

 glaring, is subdued by stirring in a little lampblack into the milk of 

 lime. 



Use of Lime as Manure.— Lime is necessary to plants. Crystals 

 of oxalate and carbonate of lime exist in the cell-walls to which they 

 impart rigidity. In its absence farm crops languish, young plants 

 stop growing, the roots die. Liming the soil has for a long time been 

 largely applied, for lime spread on the soil, especially on heavy ground, 

 produces heavier crops of potatoes, peas, beans, tares, clover, lucerne, 

 ■etc. Moreover, lime destroys the noxious weeds, which require a soil 

 poor in or almost deprived of lime and sometimes even acid. The 

 role played by lime in the practice of farming may be explained thus : 

 When lime is applied to the laud it neutralizes the acidity of the soil, 

 chiefly humic acid, which it converts into humate of lime ; the fer- 

 ments in the soil which do not work in acid lands then develop in the 

 slightly alkaline medium created by the lime ; the organic matter is 

 ■disintegrated and decomposed ; its nitrogen is converted first into 

 ammonia and then into nitric acid, the presence of which is necessary 

 to the nutrition of cultivated plants. Lime by rendering the nitrogen 



