50 CEMENTS. 



composed of a mixture of lime, sand, and water, in, 

 convseqnence of the facility with which they pass from 

 a soft state to a stony hardness, have, in common use, 

 superseded all others. Lime, in the state of quicklime, 

 is obtained by burning in kilns any of those natural 

 bodies in which it exists in combination with carbonic 

 acid ; such as limestone^ rtiarhles, chalk, and shells. 

 The effect of the burning, or calcination, is to drive off 

 the carbonic acid. If quicklime, thus obtained, be wet 

 with water, it instantly swells and cracks, becomes ex- 

 ceedingly hot, and at length falls into a white, soft, 

 impalpable powder. This process is denominated the 

 slaking of the lime. The compound formed is called 

 a hydrate of lime, and consists of about three parts of 

 lime to one of water. When intended for mortar, it 

 should immediately be incorporated with sand, and 

 used without delay, before it imbibes carbonic acid 

 anew from the atmosphere. Lime, thus mixed with 

 sand, becomes harder, and more cohesive and durable, 

 than if it were used alone. It is found that the sand 

 used in common mortar undergoes little or no change ; 

 while the lime, seemingly by crystallization, adheres to 

 its particles, and unites them together. Cements com- 

 posed in this manner continue to increase in strength 

 and solidity for an indefinite period, the hydrate of 

 lime being gradually converted into a carbonate. The 

 sand most proper to form mortar is that which is 

 wholly silicious, and which is sharp, that is, not having 

 its particles rounded by attrition. 



Fresh sand is to be preferred to that taken from the 

 vicinity of the sea-shore, the salt of which is liable to 

 deliquesce and weaken the strength of the mortar. 

 The proportions of the lime and sand to each other are 

 varied in different places ; the amount of sand, however, 

 always exceeds that of the lime. The more sand can 

 be incorporated with the lime the better, provided the 

 necessary degree of elasticity is preserved j for the ce- 

 ment becomes stronger, and it also sets, or consolidates 



