390 REPORT OF THE CHEMIST OF THE 
3 
magnesium compounds affects the slaking properties of lime, 
as pointed out below. 
Oyster-shell lime. Oyster-shells contain 90 to 95 per ct. of 
calcium carbonate and, when properly burned, produce a high- 
gerade lime, containing from 85 to 95 per ct. of calcium oxid. 
In addition to the preceding forms of commercial lime, one 
other of less importance may be mentioned, known as gas- 
lime. Quicklime is used at gas-works to remove impurities 
from the gas and after being so used it is often sold to farmers. 
Gas-lime is not quicklime, since the calcium oxid is largely 
changed into other forms such as hydroxid (slaked lime) and 
carbonate. In addition, there are the impurities absorbed 
from the gas, some of which are injurious to seeds, such as 
sulphids and sulphites. These are changed into the harmless 
form of sulphate (land plaster) on exposure to the air. Gas- 
lime should, therefore, when used on soils, be allowed to lie 
exposed to the air before use or should be put into the soil some 
weeks or months before putting in a crop. The composition 
of gas-lime varies greatly. 
SLAKED LIME. 
When quicklime undergoes the change known as slaking, 
its calcium oxid (CaO) combines with water (H,O) and is 
changed into a compound known chemically as calcium 
hydroxvid (CaO.H.,) or, commercially as slaked lime, caustic 
lime, or hydrated lime; one pound of calcium oxid combines 
with about one-third of a pound of water and produces one 
and one-third pounds of calcium hydroxid (slaked lime). This 
chemical change is accompanied by marked physical changes; 
the lime swells in bulk, crumbles to a fine powder and gen- 
erates heat. The rapidity with which commercial lime slakes 
depends upon the amount of water used and the composition 
of the quicklime. The use of too much water or the sudden 
chilling of partially slaked lime by cold water results in the 
production of a coarse, granular lime in place of the desired 
form of fine powder. Commercial limes containing less than 
10 per ct. of impurities slake more quickly than those con- 
