368 OF CALCINED AND SLAKED MAGNESIA. 



formed, during spontaneous slaking, becomes changed into carbonate 



during the slaking of the rest. But, when it has all completely fallen, 



the rapidity of the absorption ceases, and the fine slaked lime consists of 



Carbonate of lime 57*4 



Hydrate ofli^e I i;™;^^: ; ; ^l 42-6 



100 

 or, a ton of lime, left in the open air till it has completely fallen to 

 powder, contains about 8i cwt. in the state of hydrate. If left to slake 

 in large heaps, the lime in the interior of those heaps will not absorb so 

 much carbonic acid till after the lapse of a very considerable time. 

 More caustic lime (hydrate) also will be present if it be left to slake, as 

 is often done for agricultural purposes, in shallow pits covered with sods, 

 lo defend it from the air and the rains. 



After the lime has attained the state above described, and which is a 

 chemical compound* of carbonate with hydrate of lime, the further ab- 

 sorption of carbonate acid from the air proceeds very slowly, and is only 

 completely elTected after a very long period. 



2°. When slaked in the ordinary way lime falls lo powder, without 

 having absorbed any notable quantity' of carbonic acid. Numerous 

 small lumps also remain, which, though covered with a coating of hy- 

 drate, have not themselves absorbed any water. The absorption of 

 carbonic acid by this slaked lime is at first very rapid, — so that where 

 the full effect of caustic lime upon the soil is required, it ought to be 

 ploughed in as early as possible, — but it gradually becomes more slow, 

 a variable proportion of the compound of carbonate and hydrate above 

 described is formed, and even when thinly scattered over a grass-field, 

 an entire year may pass over without effecting the complete conversion 

 of the whole into carbonate. 



3°. Calcined or burned magnesia, whether in the pure stale or mixed 

 with quick-lime, as in the magnesian limes, absorbs carbonic acid more 

 slowly — and by mere exposure to the air will probably never return to 

 its original condition of carbonate. 



When allowed to slake spontaneously, three-fourths of it become 

 ultimately changed into carbonate, and form a compound of hydrate 

 and carbonate which is identified with the common uncalcined magne- 

 sia of the shops. This compoundf consists of 



Carbonate of magnesia 69*37 



Hydrate of magnesia 16-03 



Water 14-60 



100 

 and it undergoes no further change by continued exposure to the air. 

 But if slaked by the direct application of water, magnesia, like lime, 



* This compound consists of one atom of carbonate of lime (Ca O + CO2) combined with 

 one of hydrate (Ca O + HO,) and is represented shortly by Ca C -|- Ca H— in which Ca 

 denotes calcium (Lee. IX., § 4,) Ca O or (Ja oxide of calcium or lime, CO2 or C carbonic 

 acid (Lee. IIL, § I,) and II O or H water (Lee. IL § 6.) 



t It s represented by the formula 3 (Mg C + H) + Mg H. 



