When quicklime 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 
consists, as has been stated before, of 17 parts of 
water to 55 parts of lime* When hydrate of lime 
whilst it is consolidating is mixed with red oxide 
of iron, alumina, or silica, the mixture becomes 
harder and more coherent than when lime alone is 
used ; and it appears that this is owing to a certain 
degree of chemical attraction 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. 
Puzzolanais composed principally of 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 lighthouse, used a cement composed of 
equal parts by weight of slacked lime and puzzo- 
lana. Puzzolana is a decomposed lava. Tarras, 
which was formerly imported in considerable quan- 
tities from Holland, is a mere decomposed basalt: 
two parts of slacked lime and one part of tarras 
forms the principal part of the mortar used in the 
great dykes of Holland. Substances which will 
answer all the ends of puzzolana and tarras are 
abundant in the British islands. An excellent red 
tarras may be procured in any quantities from the 
Giants’ Causeway in the north of Ireland : and 
u 4 
