96 
- two or three coats of Pyrites. I have therefore exhibited it 
for its novelty, as it will very well show the nature of the 
Clay at the same time. As I found a species of Arca, new 
to me, I adda figure, as I believe they are not uncommon 
there, though not, to my knowledge, elsewhere; therefore 
they may identify something of the geology of the spot, as 
well as add to our information in this department. 
It somewhat depends on the nature of the ore to vary the 
means of obtaining the Alum from it. It is mostly burnt 
and well watered, and to the water, when saturated, is 
added Muriate of Potash, or some other Salt, sometimes 
an Ammioniacal Salt; it is then left to crystallize, which 
the Alum usually does in masses terminated by four-sided 
pyramids. In the last act of crystallization it forms the 
other half of the octaédron more or less perfectly with 
truncations, and sometimes crystals so piled on each other, 
side ‘by side, that the workmen call them Alum towers, 
making handsome ornaments under a glass; but these 
should not be exposed to the sun, nor to much warmth, as 
they will then lose their beautiful transparency. 
The Alum stone of Tolfa* in Italy is found to contain 
By Klaproth, By Vauquelin. 
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Sulphuric Acid... 16°35 |e. ay (25D 
W ater os) bliss aie y SU; On es epeeee Ree 
162°0 ~ ¥00:00 
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The Roman Alum is sometimes found in small crystals 
of a reddish cast, and is well known in shops. The nature 
of the British Alum might infer its containing some Am- 
-monia, otherwise it differs only a little in the proportions, 
and in containing a slight foreign mixture, as it is little 
else than Clay and Pyrites. Sometimes particles of Mica 
are found in it. 
Clays, which seem too common almost to claim atten- 
ticv, whether mixed with the spoils of the most precious 
rocks, plants, or animals, or with more humble materials, 
are still enriched to serye some future purpose ; and according 
to Shakespeare, at the worst, it may, like the noble dust of 
Alexauder, still serve to close a bunghole, or, like ‘* Im- 
perial Cesar, dead and turn’d to Clay, might stop a hole 
to keep the wind away.” 
* This, as may be inferred from the analysis, requires no addition of 
Alkali to make it crystallize. It seems very different from the British, 
