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There are two modes in which lime acts as a ce- 

 ment; in its combination with water, and in its combi- 

 nation with carbonic acid, 



The hydrate of lime has been already mentioned. 

 When quick lime is rapidly made into a paste with 

 water, it soon loses its softness, and the water and the 

 lime form together a solid coherent mass, which con- 

 sists, as has been stated before, of 1 7 parts of water to 

 55 parts of lime. When hydrate of lime whilst it is 

 consolidating is mixed with red oxide of iron, alumina 5 

 or silica, the mixture becomes harder and more co- 

 herent than when lime alone is used; and it appears 

 that this is owing to a certain degree of chemical at- 

 traction between hydrate of lime and these bodies; and 

 they render it less liable to decompose by the action 

 of the carbonic acid in the air, and less soluble in 

 water. 



The basis of all cements that are used for 

 works which are to be covered with water must be 

 formed from hydrate of lime; and the lime made from 

 impure limestones answers this purpose very well. 

 Puzzolana is composed principally ot silica, alumina, 

 and oxide of iron; and it is used mixed with lime to 

 form cements intended to be employed under water. 

 Mr. Smeaton, in the construction of the Eddystone 

 light house, used a cement composed of equal parts by 

 weight of slacked lime and puzzolana. Puzzolana is 

 a decomposed lava. Tarras, which was formerly im- 

 ported in considerable quantities from Holland, is a 

 mere decomposed basalt: two parts of slacked lime and 

 one part of tarras forms the principal part of the mor- 



