36 ANNUAL REPORT 
His results are given in the following table: 
Temperature. 500°C. | 650°C. | 680°C. | 790°C. 
Per cent. of Caletum Oxide in 
limestone burnt in steam....... 0 T% 23% |. 100% 
Per cent. of Caleium Oxide in 
limestone burnt in air......... 0 0 0 37% 
The impurities contained in limestone accelerate the rate of decom- 
position, this being especially true of silica. At 1000° C. 8 per cent. of 
silica causes the lime to be overburnt owing to the formation of a calcium 
silicate. The higher the temperatures the greater will be the effect of any 
silica present. The same thing may be said of alumina and iron, though 
these are less energetic in their behavior. Pure lime burnt from calcium 
nitrate at 1300° C. hydrates extreme'y slowly; burnt at 1600° C., the 
calcium oxide is crystalline and can be hydrated, but not slaked, showing 
hydraulic properties. Somewhat below 1600° C. it forms a porcelain-like 
mass of great density. In the electric furnace calcium oxide sublimes at a 
temperature corresponding to about 3000° C., melting and boiling between 
the electrodes. The condensed sublimed matter forms beautiful crystals of 
calcium oxide. The thermal reaction of the decomposition of calcium 
carbonate is as follows: 
CaCO,+437.41Cal.=CaO+CoO,. 
or I gram of calcium carbonate requires 437.4 eram calories for its 
decomposition. One pound of the carbonate of lime would thus require 
the heat of 0.06 pound of average Ohio coal, taking 7,000 calories as the 
calorific value of the fuel. 
The calcium oxide, as we know, on being brought in contact with 
water eagerly combines with the latter, forming calcium hydrate accord- 
ing to the reaction: 
CaO+H,O=—Ca(OH),+15100 cals. 
This is seen to be an exothermic reaction, one gram of the calcium 
oxide evolving 269.6 calories. The hydration is accompanied by an in- 
crease in volume. While the specific gravity of the calcium oxide varies 
from 3.08 to 3.18, that of the hydrate is 2.078. The increase in apparent 
volume of the oxide on being changed to the hydrate is about 3% times 
the original volume if just enough water is added. 
This explains the injurious action of lime in cement or bricks and 
other clay ware, the body containing the lime being destroyed by the 
hydration as soon as water is being absorbed. ‘The increase in the volume 
of the oxide depends to a great extent on the manner of slaking; that is, 
on the amount of water used. Lime hydrate is decomposed into calcium 
