374 Foreign Literature and Science. 
contrary are called meagre, which contain notable portions 
of sand or silex, alumine and iron. The former when burn- 
ed, slacked, and made into paste, will retain their softness 
for ages nadie water, or excluded from the air, but exposed 
to the air, they contract a remarkable hardness by the double 
effect of dessication and union with the carbonic acid of the 
atmosphere. They even become susceptible of a beautiful 
polish. . 
But the meagre lime stones, in general, treated inthe same 
manner, if kept under water, harden in a few days and at 
length form a kind of free stone which could be acted upon 
or broken only by the pick axe. Exposed to the air it ac- 
Pe ie a crumbly consisience and will never admit of polish. 
rom this circumstance the lime which possesses the quality 
last mentioned, is called hydraulic lime. But some of the 
meagre lime stones are unfit for hydraulic purposes, espe- 
cially those which contain large particles of silex. 
zolanas are either natural or artificial. The natural is 
found in situations which have been acted upon by subterra- 
neous heat. | _ They all consist of silex, alumine, oxide 0 
ime, sof which vary greatly. 
Silex aalereys the predominating ingredient, the lime and 
iron are sometimes, though rarely, wanting. The scoria of 
forges and furnaces, broken pottery, and pulverized brick or 
tile are artificial substances, analagous to puzzolanas. 
There is one class of puzzolanas which dissolve readily 
in sulphuric acid, and abandon the silex which immediate 
subsides. _Others resist the action of this acid, 
pidly. 
Hydraulic lime presents phenomena nearly the reverse. 
That is to say, it furnishes good mortar when combined we. 
