Factitious Puzzolana , 247 
proportions of the puzzolana and the lime, I could al- 
ways be assured of the solidity of my cements. 
There remains for me to describe the exterior characters 
of the quartziferous ferruginous oxides, which form the 
basis of my factitious puzzolana, and to relate the analy- 
sis of them which I made about eighteen years ago. I 
will content myself with offering the comparative results 
with the Italian puzzolana, both in tho dry way and the 
moist. 
Exterior Characters of the quartziferous Oxides of 
Iron . 
Their colour is of a reddish-brown before calcination, 
or slightly violet. A light torrilication gives them a 
clearer red tint or a deep violet : one more intense ren 
ders them of a deep brown or of a violet-brown inclining 
to a black. The degree of the calcination for use is con- 
fined to those two states. 
Urged at a longer continued heat, the colour becomes 
a deep black ; then the substance becomes porous, entire- 
ly similar to certain lavas of our modern or ancient vol- 
canoes, with which it is then difficult not to confound 
them. 
Their fracture is grained and a little earthy, and 
small crystals of quartz may be distinguished in them 
by the naked eye, and almost always angular fragments 
of gray or milky quartz; a powerful magnifying lens 
causes in some fragments the discovery of needles of 
schorl, the amphibole of Hauy, and some small tourma- 
lines. 
Their smell is strongly argillaceous on breathing on 
them with the mouth. 
There is no fire produced by the use of the steel, when 
it does not strike a quartzose particle. 
They do not effervesce with acids either cold or hot , 
