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728 VON TROIL’s LETTERS ON ICELAND.
earth has not before been enough united with the filiceous earth, at leaft not with the
whole of it. Itis well known that lime and quarz, when expofed to the operation of a
fire, that has only caufed them to bake together, neverthelefs afterwards yield a gelatin-
ous fubftance. Alcaline falt mixed with fand affords a fimilar demi-concretion, as
when we calcine pot-afhes; on which account the cleareft folution a long while after
precipitates filiceous powder, in the fame proportion that the alcaline falt attraéts fixed
air, with which it preferably ‘unites itfelf. This generally produces a gelatinous con-
gelation, when the water is faturated with alcaline falt, and alfo is well charged with
filiceous earth.
For the fame caufe, clay, fpathofe fluor, and other fubftances, hard to be diffolved in
acids, may be brought to a gelatinous congelation, when they have before by fufion
been united with alcaline falt, borax, or calcareous earth: calcareous earth by itfelf
never gives a gelatinous fubftance in acids, confequently it can fo much the lefs become
a filiceous earth by this method, as has however been thought by fome, who would foon
relinquifh their opinion if they would only make experiments themfelves ; where there
isno flint before, it cannot poflibly be propofed by any other acid, but that which is
obtained from fpathofe flour.
But at prefent this is enough of the zeolite, of which I have treated more extenfively,
asit is found in great abundance in Iceland, and is f{uppofed by fome to be produced in
the dry way ; but I hope that this matter is at prefent entirely determined, not only by —
its {eparation, but alfo by its production, which happens daily in the water. See N> IL.
I have for feveral years paft endeavoured to difcover the number as well as the nature
of the original kinds of earth. In the year 1758 Mr. Cronftedt counted nine ; if he
had lived longer for the benefit of the fciences, he would no doubt have reétified this
account. In confequence of my experiments I have difcovered the following fix
forts :
1. Calcareous earth, which after being faturated by acid of vitriol affordsa kind of
ypfum.
aE Terra ponderofa, which with acid of vitriol gives a ponderous fpar, and in feveral
refpects is very different from the calcareous earth. ‘
3. Magnefia, which, together with acid of vitriol, produces the Englifh or Epfom
falt.
‘Thefe three kinds are generally found faturated with fixed air, and they are on that
account fubjeét to an effervefcence with ftronger acids.
4. Argillaceous earth, which, together with the vitriolic acid, produces allum.
‘The common argillaceous earth is always mixed with filiceous earth, but the fort here
nicant mutt be entirely pure.
5- Siliceous earth, which is not attacked by any acid yet known, the flour acid ex-
cepted. In the dry way it can be diffolved with a third part of its weight of fixed alkali
to a tranfparent lafting glafs, which at firft is affected witha ftrong effervefcence.
6. Gemmceous earth, that is not attacked by any known acid, and clearly diftinguifhes
itfelf from the preceding forts, by its being entirely indifloluble, and being fubje& to a
weaker effervefcence in the fire with fixed alcali. It is found in all the gems or pre-
cious ftones.
I have in vain made ufe of various methods to feparate thefe earths into more fimple
principles, and to all appearance others would have no better fuccefs than I have had :
it they are really compounds, they are at leaft fimple in regard to the method known
among us of feparating fubftances, and do not arife from one another. Whatever has
therefore been objected to this opinion from prejudice, cannot fubfift after experiments
4 have
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