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 under 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- 
quires a crumbly consistence and will never admit of polish. 
From 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, pape- 
cially those which contain large particles of silex. 
-Puzzolanas 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 of 
iron, and a little lime, the properties of which vary greatly. 
Silex is always 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 immediately 
subsides. Others resist the action of this acid. 
‘If we mix in various proportions, very rich lime, slacked 
in the usual way with sand alone, or with puzzolana which 
resists the action of sulphuric acid, we obtain a mortar, 
which, placed under pure water, remains always soft, or ac- 
quires, after a long time, only a feeble consistence. The 
same mortar exposed ‘to the air, soon hardens by drying, 
but is always easily broken or pulverized. But if the same 
experiment is made with a puzzolana-readily affected and 
decomposed by sulphuric acid, a mortar is obtained, which 
soon ses under water, and becomes gradually harder, but in 
air it does not acquire any great resistance in saa pa 
of its drying too rapidly. 
Hydraulic lime presents phenomena nearly the reverse. 
That is to say, it furnishes good mortar when combined with 
