222 
CHEMISTRY. 
fa Its, fome infallible, fame very foluble. Its action upon 
filex, alumine, and glucine, has not yet been afcertained. 
The compofition or component parts of zircone are riot 
known. 
Magnesia. —This earth is never found pure and na¬ 
tive ; iome affirm, however, that it is found in complete 
maffes in Savoy. It is met with either combined chemi¬ 
cally with different acids, or mechanically mixed with ter- 
reftrial fubftances, fuch as talcs, fteatites, foap-rock, fer- 
pentines, lapis cllaris, afbeftos, mountain-cork, carbonat 
©f magnefia and of lime, fulphat of barytes, alumine, 
and iron: this combination forms amianthus, and all 
the calcareous ftcnes. 
To obtain it pure, diffolve in diftilled water cryftalsof 
fulphat of magnefia, orEpfom fait, which is to be afterwards 
deeompofed by carbonat of potaffi. Wafh well the pre¬ 
cipitate, and calcine it to difengage the carbonic acid. 
Pure magneiia is in the form of a very fine white powder, 
or in little brittle fragments; it has no fer.fible tafte, but 
it dries and leaves a flight fenfation of bitternefs on the 
tongue. It purges gently the ftomach and bowels. It 
turns fyrup of violets and mallows of a greenifh colour, 
and turnfole to a blue. It is not altered by a common 
fire ; but in a very ftrong heat becomes brittle or friable. 
Heated in a retort, it acquires a phofplroric property. 
Expofed to the air, it infenfibly takes in carbonic acid, 
and becomes effervefcent. It requires near 2000 parts of 
water to diffolve it. With acids, it forms very loluble 
falts. Guyton has made feveral experiments in the way 
of combining magnefia with other earths : he produced a 
kind of vitrification. Magnefia has hitherto been ufed 
only in medicine : it afts in various manners, according 
to the Hate in which it is given ; whether crude, calcined, 
or combined with acids. 
Lime. —Though lime, otherwife called calcareous earthy 
(from calory Lat. heat,) be met with almoft every where 
in the bowels of the earth mixed with carbonic acid, it 
requires a procefs to obtain it pure. For this end, let 
chalk be waflied in diftilled boiling water; then diffolve 
it in diftilled acetous acid, and precipitate it by means 
of carbonat of ammoniac, wafh the precipitate, calcine 
it, and the refidue is pure lime. 
Another method is to extract it from oifter-fhells, as 
follows : Wafh the ffiells in feveral waters, then boil them 
to remove the mucilage which generally adheres to them; 
the water diffolves fome faline particles alfo. Put them 
into a furnace, and calcine them to wdiitenefs. After 
this firft calcination, put them into a retort of earth or 
porcelain, and make them red-hot. The refult is very 
pure lime, called quicklime. Topreferve this, put it in 
glafs veflels well flopped, and it will keep without altera¬ 
tion for years. 
In preparing lime for ufe in the arts, it often happens 
that it is burnt; it then has no longer the qualities of 
quicklime, but is covered with a kind of plalter like an 
imperfedt vitrification. 
Calcareous earth, deprived of water and acid, and re¬ 
duced to a fimple ftate, is a whitifh fubftance; its tafte 
is urinous, acrid, hot, almoft cauftic; it gives fyrup of 
violets a deep green colour, and makes no effervefcence 
with acids. Expofed to the air, it attradls the humidity 
of-the atmofphere as it comes into the ftate of Jlaked lime: 
this moifture caufes it to lwell, break, and fall to powder; 
its weight is increafeft, and its union with the carbonic 
acid which it has drawn from the atmofphere makes it 
effervefcent: it repaffes infenfibly into the ftate of calca¬ 
reous earth, and from quicklime becomes Jlaked lime. The 
effects are quicker and more remarkable when the atmof¬ 
phere is very moift. Heat is excited during this flaking 
in the air, or dry flaking. It is attended with heat; for 
it contains water in a dry aud fblid ftate, and the ther¬ 
mometer rifes in it to 120“. The dilatation is fufficiently 
ft rots g to burft cafks or other wooden velfels in which 
lime is contained. 
It is ealy to deprive flaked lime of the moifture and the 
acid it at trailed from the air. To this end put fomer 
flaked lime into a gun-barrel, or an earthen retort; bring, 
it to a red heat, and the lime returns to its original ftate 
of quicklime. 
The adlion of water on quicklime is very remarkable*- 
When a fmall quantity of this fluid is poured on lime, 
it is quickly ablorbed, the lime appearing as dry as be¬ 
fore : after a fhort interval of time, it burits into pieces,, 
producing a degree of heat fufficient to reduce the water 
into vapours, with a remarkable hiffing noife. Thefe va¬ 
pours have a peculiar fmell, and give a green tinge to 
paper ftained with mallows: the lime foon falls into a 
white powder; the heat, the agitation, and the vapours, 
gradually dilappearing. If this extinilion be made during 
the night, or in a dark place, many luminous points are 
obferved on the furface of the lime. All thefe pheno¬ 
mena are confequences of the activity with which this 
falino-terreflrial fubftance unites with water; but, in or¬ 
der that they may take place, it is required that no more 
water be ufed than the lime can very quickly abforb, fo 
as to become immediately dry. It feems that the difen- 
gagement of heat from thefe two bodies during this rapid 
union changes their ftate, and that flaked lime in its pul¬ 
verulent form, contains water in a dry and folid ftate. 
This dry ftate of water, which takes place in many com¬ 
binations, attended with heat, and which produces folid 
compounds, whole fpecific heat is lefts coniiderabie than 
before, has not been enough attended to by chemifts, or, 
to l'peak more properly, has been totally unobi'erved till 
lately. When lime, in this experiment, has ablorbed as 
much water as it can combine with and remain dry, it is 
called dry flaked lime; it then no longer produces heat 
by the addition of water, but is diffolved without any 
fenfible motion. By mixing with this fubftance the quan¬ 
tity of water neceffary to dilute it, milk of lime is produced; 
if the quantity of water be Hill greater, the lime is per- 
feftly diffolved, and the liquor becomes tranlparent. Mr. 
Kirwan affirms, that 680 parts of water are required to 
diffolve one of lime, at the temperature of 60 degrees. 
This folution, which is known by the name of lime-nvater y 
is clear and limpid ; its fpecific gravity fcarcely exceeds 
that of common water; its tafte is acrid and urinous ; 
and it readily converts fyrup of violets to a green, and 
even deftroys the colour.. "By evaporation in doled veffels, 
very pure water is obtained, the quicklime remaining be¬ 
hind; but a red heat is neceffary to feparate the laft por¬ 
tions of water, which are retained with great force : after 
this treatment, the lime becomes heated by the addition 
of fmall quantities of water as before. 
Lime-water expoied to the air becomes covered with 
a dry pellicle, which gradually increafes in thicknefs and 
folidity : if this pellicle be taken away, a fecond is form¬ 
ed, and after that a third, and fo forth, till the whole of 
the water is evaporated. Thefe pellicles have been im¬ 
properly called cream of lime ; it was formerly thought to 
be a peculiar fait formed by the union of the moft lubtle 
part of the calcareous earth united to water; and much 
has been written concerning this pretended fait of lime. 
But it is now admitted, fince the experiments of the ce¬ 
lebrated Dr. Black, that the faline properties of cream of 
lime are lefs intenfe than thofe of the lime itfeif; and 
that it is a peculiar neutral fait compofed of lime, and 
an acid extracted out of the atmofphere. Hence it is 
that the cream of lime cannot be formed without the 
contafl of the air. The fait is, that the lime draws car¬ 
bonic acid frpm the atmofphere, as mentioned before, 
and forms on the furface, at its diffolution, a cruft of 
chalk, or of calcareous carbonat. 
Lime unites eaffiy with acids, forming falts, fome in- 
foluble, others very eafily foluble. It combines with fili- 
ceous earth in the humid as well as in the dry way. 
When fand is mixed with lime newly flaked, or with 
quicklime fprinkied with a fmall quantity of water at 
the time of mixing, thefe two bodies become confident, 
and form what is called mortar. The ftate and quantity; 
