LIME. 



304 



LIME. 



purposes is derived, consist of calcic car- 

 bonate and carbon dioxide in the propor- 

 tion of 14 parts of the former to 1 1 parts of 

 the latter. When subjected to the action 

 of fire in a kiln, the carbon dioxide is 

 liberated and driven off in the form of 

 carbonic acid gas, a shimmering vapour 

 that may be seen rising from the mouth of 

 any kiln that is alight, and leaves the lime 

 behind it in a pure or nearly pure state. 

 In this condition it is known as caustic or 

 quick lime. When water is poured on it, 

 it heats almost immediately, pours forth 

 steam, and ultimately falls to pieces, 

 assuming the form of a white powder, 

 which is known as slaked lime, the opera- 

 tion of reducing the lime to powder by the 

 agency of water being termed slaking. By 

 chemists slaked lime is called hydrate of 

 calcium. The powder thus obtained is 

 composed very nearly of 3 parts of lime 

 to I part of water. If quicklime is exposed 

 to the action r.i the air, it first takes in 

 water from the atmosphere, and then falls 

 to powder in the same way as it does when 

 water is poured directly upon it, though 

 not so quickly. Lastly, the slaked lime, 

 whether converted into powder rapidly by 

 water or slowly by the air, gives off the 

 water that it has retained, absorbs carbonic 

 acid from the air, and ultimately becomes 

 calcic carbonate, reverting as it were to the 

 state in which it existed before it was 

 burnt in the kiln ; the advantage arising 

 from burning being chiefly that it is brought i 

 into a state and condition suitable for ad- 

 mixture with the soil. 



Advantages of Burning Lime. The ad- 

 vantages of burning lime are thus set forth 

 in the " Elements of Agricultural Chemistry 

 and Geology," by Johnston and Cameron: 



" If the lime turn to the same chemical 

 state of carbonate in which it existed in 

 the state of chalk or limestone, what is the 

 benefit of burning it ? The benefits are 

 partly mechanical and partly chemical. 



'* (a) We have seen that on slaking the 

 burnt lime falls to an exceedingly fine 

 bulky powder. When it afterwards be- 

 comes converted into carbonate, it still 

 retains this exceedingly minute state of 

 division ; and thus, whether as caustic 

 hydrate or as a mild carbonate, can be 

 spread over a large surface, and be in- 

 timately mixed with the soil. No available 

 mechanical means could be economically 

 employed to reduce our limestones, or even 

 our softer chalks, to a powder of equal 

 fineness. 



" (/>) By burning, the lime is brought 

 into a caustic state, which it retains, as we 

 have seen, for a longer or shorter period, 

 till it again absorbs carbonic acid from the 

 air or from the soil. In this caustic state, 

 its action upon the soil and upon organic 

 matter is more energetic than in the state 

 of mild lime ; and thus it is fitted to pro- 

 duce effects which mere powdered lime- 

 stone or chalk could not bring about at all, 

 or to produce them more effectually and in 

 a shorter period of time. 



" (c) Limestones often contain sulphur in 

 combination with iron (iron pyrites). The 

 coal or peat wif.h which it is burnt also 

 contains sulphur. During the burning, a 

 portion of this sulphur (oxidised) unites 

 with the lime to form gypsum, by this 

 means adding to the proportion of this 

 substance which naturally exists in the 

 limestone. 



"(d) Earthy and silicious matters are 

 sometimes present in considerable quanti- 

 ties in our limestone rocks. When burnt 

 in the kiln, the silica of this earthy matter 

 unites with lirne to form calcic silicate. 

 The silicate being diffused through the 

 burnt and slaked lime, and afterwards 

 spread in a minute state of division over 

 the soil, is in a condition in which it may 

 yield silica to the growing plant, supposing 

 silica to be essential. 



" Thus the benefit* of burning are, as 



