Factitious Puzzolana, 
solidification in the water was owing to the good quality 
of the lime. 
I was thus obliged to recur to my quartzose oxides of 
iron, from their containing a greater quantity of ferrugi- 
nous principles; and can aver with the skilful Faujas, 
that the puzzolanas owe their property of hardening in 
water solely to the ferruginous particles which they con 
tain : of this I have had many proofs. This truth is far- 
ther demonstrated in the pudding- stones, the brescias, 
and generally in all the amygdaloides with a ferruginous 
base or cement. 
The theory of our cements is but little advanced ; per- 
haps we take simple conjectures for proofs relative to 
them. We effect the regeneration of silex, and of the 
carbonate of lime ; we know the acid gases which per- 
form the principal part in the affair : but in this important 
work we have been long ignorant of the degrees of their 
reciprocal affinity, their quantity, and the mode of their 
respective combinations. Our knowledge on this matter 
is confined to a few facts. 
Many experiments have proved to me that the puzzo- 
lana, which soonest forms a body in the water, is not fit 
to be employed in the open air, where it cracks and chaps 
in all directions. And that which is proper for the air, 
and which acquires and preserves its tenacity in it, sets 
but imperfectly in water. This difficulty, of which the 
Institute will perceive the cause, has obliged me to keep 
two sorts of the factitious puzzolana; on the reciprocal 
use of which a memoir of instruction will accompany 
the sale. The two sorts may be distinguished by their 
colour. 
The factitious puzzolana proper for works under wa- 
ter, is of a reddish-brown. That which is fit for works 
exposed to the air, is a dark violet. The latter is used 
for terraces, the embankments of basons, for the com 
