SULPHATE OP LIME. * 111 



tion far less noisome than it is under ordinary cir- 

 cumstances ; because the lime absorbs sulphuretted 

 hydrogen, carbonic acid, and many of the products 

 of decay. 



235. When quicklime is slaked with water, it crum- 

 bles down to a powder, and is found to have combined 

 with a quantity of water ; although it appears quite 

 dry, for the water is chemically combined with the 

 lime : if this lime thus slaked is left exposed to the 

 air, it combines with carbonic acid, and becomes 

 changed into carbonate of lime, and at the same time 

 parts with the water with which it had previously com- 

 bined. Slaked lime is less strongly caustic than quick- 

 lime, because part of the action of lime on organic 

 matter is caused by its strong affinity for water ; 

 nevertheless, even slaked lime possesses strong caustic 

 properties. 



236. Common mortar, which consists of slaked 

 lime mixed with sand, chopped hair, &c., is a hydrate of 

 lime ; its setting depends chiefly on the absorption 

 of carbonic acid from the air, which forms again with 

 it a kind of chalk or limestone. Hence mortar gradu- 

 ally improves, becoming harder after a time, when it 

 has absorbed a sufficient quantity of carbonic acid 

 from the air to be wholly converted into carbonate. 



237. Sulphate of lime is less common and abund- 

 ant than the carbonate ; it is, however, a frequent in- 

 gredient in the soil, in spring and mineral waters, and 

 is found sometimes in large beds, constituting what 

 is called plaster-stone, gypsum, and alabaster ; these 



