252 
On natural and artificial Puzzolana. 
ments, that they might he employed with great advantage 
in buildings under water. 
The Dutch terrass is a kind of pumice stone brought 
from Bonn and Andernach. At Dordrecht, at the mouths 
of the Rhine and Meuse, the operation of pounding is 
effected. 
But these resources are local; and as the manufacture 
of puzzolana may become general, we proceed to describe 
the best means of attaining it. 
It would be difficult to assign the period at which 
pounded bricks and the earthy residue from the distilla- 
tion of aquafortis were substituted for volcanic puzzola- 
na. Their use, however, has become general, particular- 
ly where there are no sea-ports in the vicinity at which 
real puzzolana can be furnished : even in the south of 
France they prefer the earthy residue of the distillation 
of aquafortis to the best puzzolanas for coating the inside 
of the wine tubs* which are almost all of mason work, 
and for the cements used by individuals in hydraulic 
works. The earth employed in the south of France for 
the decomposition of saltpetre, by extracting the aquafor- 
tis from it, is an ochrey earth very much charged with 
iron, and more or less reddened by the oxide of this me- 
tal. When it is wanted for cement, it is only necessary 
to beat it up with lime and a proper quantity of water. 
M. Lepere relates some experiments made at Paris by 
the engineers of roads and bridges, from which it ap- 
pears that an immersion of eight days was sufficient 
for aquafortis cements to acquire a hardness fit to resist 
a billet of wood when forced against it with the whole 
strength of a man; whereas the Italian puzzolana re- 
quired six weeks before it attained the same degree of 
hardness. 
In general the quality of the earth is better in propor- 
tion as it is charged with iron. 
