366 OF THE m UNING AND SLAKING OF LIME. 



magnesia, and therefore, nre found to produce excellent lime for agri- 

 cultual purposes — while in others this substance forms nearly one- 

 half of the whole weight of the rock. Similar differences are found 

 to prevail in almost every locality. 



This admixture of magnesia in greater or less quantity is not con- 

 fined to the lime-stones of the magnesian lime-stone formation pro- 

 perly so called. It is found in sensible quantity in certain beds of 

 lime-stone in nearly every geological formation, and there are few 

 natural lime-stones of any kind in which traces of it may not be dis- 

 covered by a carefully conducted chemical examination. 



The simplest method of detecting magnesia in a lime-stone is to dis- 

 solve it in diluted muriatic acid, and then to pour clear lime water into 

 the filtered solution. If a hght white powder fall, it is magnesia. The 

 relative proportions in two lime-stones maybe estimated pretty nearly 

 by dissolving an equal weight of each, pouring the filtered solutions 

 into bottles which can be corked, and then filling up both with lime 

 water. On subsiding, the relative bulks of the precipitates will indi- 

 cate the respectivB richness of the two varieties in magnesia. 



§ 2. Of the huTniing and slaking of lime. 



Burning. — When carbonate of lime or carbonate of magnesia is 

 heated to a high temperature in the open air the carbonic acid they 

 severally contain is driven oif, and the lime or magnesia remains in 

 the caustic state. When thus heated the carbonate of magnesia 

 parts with its carbonic acid more speedily and at a lower temperature 

 than carbonate of lime. 



On the large scale this burning is conducted in lime kilns, one ton 

 good lime-stone yielding about 11 cwts. of burned, shell, quick, or 

 caustic lime. 



Slaking. — When this shell or quick-lime, as it is taken from the kiln, 

 is plunged into water for a short time and then withdrawn, or when a 

 quantity of water is poured upon it, heat is developed, the lime swells, 

 cracks, gives off much watery vapor, and finally falls to a fine, bulky, 

 more or less white powder. These appearances are more or less rapid 

 and striking according to the quality of the lime, and the time that has 

 been allowed to elapse after the burning, before the water was applied. 

 All lime becomes difficult to slake when it has been for some time ex- 

 posed to the air. When the slacking is rapid as in the rich limes, the 

 heat produced is sufficient to kindle gunpowder strewed upon it, and 

 the increase of bulk is from 2 to 3i times that of the original lime 

 shells. If the water be thrown on so rapidly or in such quantity as 

 to chill the lime or any part of it, the poAvder will be gritty, will con- 

 tain many little lumps which refuse to slake, will also be less bulky 

 and less minutely divided, and therefore will be less fitted either for 

 agricultural or for building purposes. 



When quick-lime is left in the open air, or is covered over with sods 

 in a shallow pit; it gradually absorbs water from the air and from the 

 soil, and falls, though much more slowly, and with little sensible deve- 

 lopment of heat, into a similar fine powder. In the rich limes the in- 

 crease of bulk may be 3 or 3i times ; in the poorer, or such as contain 

 much earthy matter, it may be less than twice. 



