FURTHER CHANGES UNDERGONE BY SLAKED LIME. 367 



-Hydrate of lime. — Wlien quick-lime is thus slaked it combines with 

 the water which is added to it, and becomes converted into a milder or 

 less caustic compound, \yhich among chemists is known by the name of 

 hydrate of lime. This'hydrate consists of 



Lime . . 76 per cent. I or one ton of pure burned lime becomes 

 Water . . 24 " \ nearly 25 cwt. of slaked lime. 



100 



It is rare, however, that lime is so pure or so skilfully and perfectly 

 slaked as to take up the whole of this proportion of water, or to increase 

 quite so much as one-fourth part in weight. 



Hydrate of Magnesia. — When calcined or caustic magnesia is 

 slaked, it also combines with water, but without becoming so sensibly 

 hot as quick-lime does, and forms a hydrate, which consists of 



Magnesia . 69-7 per cent. ? or one ton of pure burned magnesia be- 



Water . . 30-0 » \ comes 28| cwt. of hydrate. 



100 



When magnesian lime is slaked, the fine powder which is obtained 

 consists of a mixifire of these two hydrates, in proportions which depend 

 of course upon the composition of the original lime-stone. 



An important difference between these two hydrates is, that the hy- 

 drate of magnesia will harden under water or in a wet soil in about 8 

 days — forming a hydraulic cement. Hydrate of lime will not so 

 harden, but a mixture of the two in the proportions in which they exist 

 in the Hartlepool, Humbledon, and Ferryhill lime-stones (page 365), 

 will harden under water, and form a solid mass. In the minute state 

 of division in which lime is applied to the soil, the particles, if it be a 

 magnesian lime, will, in wet soils, or in the event of rainy weather 

 ensuing immediately after its application, become granular and gritty, 

 and cohere occasionally into lumps, on which the air will have little 

 effect. This prop^ty is of considerable importance in connection with 

 the further chemioal changes which slaked limes undergo when exposed 

 to the air or buried in the soil. 



§ 3. Changes lohich the hydrates of lime and magnesia undergo by 

 prolonged exposure to the air. 



When the hydrates of lime or magnesia obtained by slaking are ex- 

 posed to the open air, they gradually absorb carbonic acid from the at- 

 mosphere, and tend to return to the state of carbonate in which they ex- 

 isted previous to burning. By mere exposure to the air, however, they 

 do not attain to this state within any assignable time. In some walls 

 600 years old, the lime has been found to have absorbed only one fourth 

 of the carbonic acid necessary to convert the whole into carbonate ; in 

 others, built by the Romans 1800 years ago, the proportion absorbed 

 has not exceeded three fourths o^ ihe quantity contained in natural lime- 

 stones. In damp situations the absorption of carbonic acid proceeds 

 most slowly. 



1°. Change undergone by pure lime during spontaneous slaking. — 

 In consequence, however, of the strong tendency of caustic lime to ab- 

 sorb carbonic acid, a ejnsiderable quantity of the hydrate of lime first 



