1’1'G On natural and artificial Pitzzolanas. 
without receiving any impression, or which, loaded on a surface of 0,00005 metre 
with a weight of 2 kilogrammes, experience no perceptible depression ; 
Middling puzzolana clays, those for which it is necessary to wait a month or six 
weeks for a similar result; 
Lastly, null mixzolami clays, those which give mortars remaining soft indefi- 
nitely, and which the finger penetrates with ease. X remark that I have found clays 
of this class only among earths strongly ochreous and of the colour of wine-lees, of 
which I have spoken above ; but others may exist. 
In the first place, I should remark that, in the first class are comprised, not only 
the clays ex tracted from the energetic sands, of which X have spoken in my ii rst note, 
but also the yellowish brmcn, and other clays which are met with in nature without 
mixture of sand. As much may be said of the second class, and hence it must be 
concluded than the clays extracted from sand or gravel do not possess peculiar pro- 
perties, but rhatthese properties are common to them in the same degree with ma- 
ny other clays of different colours ; and that the properties of sands t arencs) have 
been remarked on account, of the mixture of siliceous fragments only, which, be- 
ing found ready made, renders these properties much more striking, as I shall ex- 
plain elsewhere- 
Having classed, by tliese first trials, the substances which I intended to exa- 
mine, I submitted them, in a slate of dust, to very nearly a dull red heat for fifteen 
minutes only, in an open crucible. The following are the phenomena which ap- 
peared: clays of the two classes speedily underwent a sort of ebullition; at the 
same time their colour changed rapidly, passing from yellowish red, yellow, yel- 
lowish brown, &c. to deep brown red, to bright red, to blackish red. &c. ; and 
weighing the substance with care before and after this operation, X found that 
these calcined clays had lost variable quantities of their weights^ amounting in some 
to a fifth part of their original weight. 
Clays of the last dais, on the contrary, did not perceptibly change colour, nor 
lose generally above two or three hundredths of their weight. 
By forming mortars with clays thus prepared, iu the same proportions and with 
the same lime that I had employed for trying the natural clays, X found, first, that 
clays of the first class all became, without exception, excellent puzzolanas, in other 
words, the mortars obtained as above-mentioned, and immersed, acquired In the 
course of two days sufficient consistency to resist completely an impression from 
the finger ; that these mortars tried at the end of fifteen days by the penetration jf 
a point, exhibited a degree of hardness equal to that of mortars of the same raw clay 
at the end of four months ; and that by pursuing this Comparison till a more distant 
period, the progress of the calcined clay mortars being afterward slower than that 
of the mortars of raw clay,, there was no longer any appreciable difference, at the 
end of a year, between them, it being understood that care was taken first to scrape 
the surfaces in contact with the water to the depth of one or two centimetres (=.4 
to 0.8 inch.), an operation, the necessity of which 1 have remarked in my first note 
inserted in the Annates on this subject. Secondly, that clays of the second class 
exhibited nearly the same phenomena, with this difference, that puzzolanas obtain- 
ed by calcination were generally less energetic, and afforded mortars less hard 
than the preceding. There was besides a much greater difference between these 
mortars and those of raw clay, than for clays of the first class. Tire latter required 
more than eight months to attain the degree of hardness, which the others acquired 
in fifteen days. Thirdly, clays of the third class did not appear to have gained 
any thing, or gained very little by calcination, and gave mortars which remained 
constantly soft under water as before. 
Although it appeared to me very probable, that the effect of a calcination so slight 
and of so short a continuance as that to which I had submitted the different days, 
could only have been to decompose a hydrate, and that the remarkable diminution 
of the weight of the substance, the kind of ebullition and remarkable change of co- 
lour must ba attributed to the disengagement of water retained in combination: 
yet I thought it necessary to satisfy myself directly that there was neither a disen- 
gagement nor an absorption of gas in that operation. To do this I distilled a de- 
termined quantity of clay of the first class, in tho state of dust in a retort connect- 
ed with a balloon by a tube ; I found it sufficient to keep the retort at nearly a 
dull red heat for fifteen or twenty minutes. The change of colour took place as 
■when exposed to the air, and aqueous vapour condensed in the balloon in drops. 
When the apparatus had cooled, and the water collected together the weight of the 
