On natural and artificial Puzzolana . 258 
This last observation is equally applicable to pounded 
bricks: in general they do not make a good cement 
unless they are well burnt., and made of very ferruginous 
earth. 
Twenty years ago I suggested the above substitutes for 
puzzolana ; and the result of my comparative trials made 
in the port of Cette, under the inspection of the engineers 
of the province of Languedoc, was published in 1787? in 
a memoir printed by Didot, by order of the states-general 
of the province. 
The means which I suggested for making this artificial 
puzzolana are simple, and may be put in practice almost 
every where. Balls should be made of the ochrey earth, 
and burned in a lime or potter’s kiln. In order to form 
these balls, the earth must be moistened with a sufficient 
quantity of water ; and when the balls are made, they 
should be burned until they pass from a red to a black 
colour, and the angles of the scales formed when they are 
broken exhibit sharp and shining edges. 
In the same work I proposed to substitute the blackish 
schists which are decomposed in the air for puzzolanas. 
Those which are in cakes are best; but in all cases they 
must be strongly calcined, in order to give them the ie= 
quisite properties. 
M. Lepere relates that M. Vitalis, professor of che- 
mistry and secretary to the Rouen Academy, and M. 
Lamassen, chief engineer of the department of the 
Lower Seine, have made most excellent puzzolana by 
the calcination of some ochrey earths in the environs 
of Rouen : this was effected by burning the earth in 
a common furnace with alternate strata of common char- 
coal. This puzzolana was subjected to some trials on 
a large scale, and it was composed in the following 
manner : 
One part and a half of yellow calcined ochrey earth. 
