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VON TROIL’S LETTERS ON ICELAND: 929
Laftly, feveral forts have the quality of yielding jellies, that is to fay, they change a pro-
portionable quantity of acid to a femi-tran{parent congealed mafs, which refembles a jelly.
Thefe congelations may in general be produced in different ways; fometime the men-
ftruum by length of time lofes its power, when the diflolved part is attacked gradually,
though imperteétly diffolved, fo that in a manner it remains fufpended half diffolved, and
after fome evaporation at laft congeals to a tough coherent fubftance. This frequently
happened in diffolving tin in marine acid, or aqua regia, when the inflammable fub-
{tance decreafes too much, and by that means weakens the connection between the
metal and the folvent.
Sometimes a kind of gelatinous congelation is produced by an imperfect precipita-
tion. For inftance, when the liquor filicum is united with a certain quantity of acid,
fo that the filiceous earth is not entirely feparated, but remains fufpended in the liquor.
This much refembles a circumftance exhibited by fome zeolites, which I will now more
fully explain.
As the red zeolite of Adelfors produces this effe&t more clearly than any other, it
fhall ferve as an inftance of it. After this is feparated and freed as much as poflible
from calcareous fpar, three or four tea-fpoons full of it muft be thrown into a wine
lafs half filled with common aqua-fortis; when after a fhort time the whole folution
will be found in the form of a reddifli gelatinous fub{tance, that nothing of it runs out
if even the glafs is turned. To difcover:the caufe of this, I have taken fome of the
cleareft jelly, and diffolved it with boiling water in a glafs mortar, and left it todry ona
filtering paper after a perfect filtration, by which means the place which it occupied
was incredibly diminifhed. I then tried this fubftance with acids, but it was not at all
attacked, and did not melt in the ftrongeft fire alone... The fufible urinous falt hardly
attacked it; borax diffolved it, though with difficulty ; but fal fodze diffolved it perfectly
with a ftrong effervefcence. In confequence of this the gelatinous fubltance chiefly
confifts of filiceous earth expanded in the higheft degree. But by what means has this
indifoluble fubftance been introduced into a folvent ?
We have before obferved, that the Adelfors zeolite contains eighty parts in a hun-
dred of filiceous earth, nine anda half of argillaceous earth, and fix anda half of cal-
careous earth free from fixed air ; all which fubftances are united as clofe as poflible.
If therefore the powder is thrown into an acid, and remains there during fome time,
the argillaceous and-calcareous earths are immediately attacked by it ; but thefe are in-
ternally connected with the filiceous earth, and confequently take a confiderable part of
it half diffolved into the fpungy and {wollen ftate, which all fubftances generally ex-
hibit in the moment of precipitation. ‘The fame thing happens when a refinous gym is
laid into fpirits of wine ; part of the gum, together with the refin, is then immediately
diffolved by the fpirits of wine on account of its connection, though the firft alone can-
not be diflolved by it at all. If a fufficient quantity of water or acid is added before the
liquor begins to congeal, no congelation enfues, but the filiceous earth falls in loofe
flakes to the bottom, which evidently proves that the folvent, in regard to its quantity
and ftrength, muft be confined within certain limits. An addition of fome chalk in-
creafes its tendency to gelatinous congelations, partly becaufe the folution becomes
more broken and folid, and partly by means of bubbles of fixed air, which attach them-
felves to the fpungy filiceous powder, and make it lighter ; whence the zeolite of Adel-
fors in this cafe feems to have an advantage over every other fort, on account of the
~ lime-fpar naturally mixed with it; the principal part however no doubt depends on the
feveral parts which compofe it. Some forts afford onlya gelatinous fubitance after a
preparatory calcination ; the caufe of which moft probably is this, that the calcareous
iI : earth
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