MAGNESIATA. 
ten not without trituration ; becoming black in the fire. 
Profeflor Kidd obferves, that its natui'al compounds are, 
principally mixtures, or chemical combinations, of dif¬ 
ferent earths with oxyd of iron ; and, generally fpeaking, 
are remarkable for an almolt uiifluotis foftnefs, percepti¬ 
ble upon touching them ; and for their difpofition to a 
fibrous texture, and green colour. But, though thefe cha¬ 
pters evidently depend upon the prefence of magnefia, 
the proportion of this earth rarely amounts to fo much as 
one half of the weight of the mineral in which it is con¬ 
tained. Of the other component parts filex is the moll 
abundant, and frequently amounts to as much as all the 
left. . 
1. Magnefiata granulans, common magnefia, or brown 
fpar : fubopake, tranquil in the fire, breaking into inde¬ 
terminate fragments of a common form. Found in va¬ 
rious parts of Germany, Sweden, France, Sec. in large 
mafles ; colour white, flefli and rofe-colour, greyilh, yel- 
lowifh, and reddifh-w.hite, with frequently an iridefeent 
metallic appearance ; generally opake, and becomes 
brownifh when expofed fome time to the air. Contains 
carbonat of lime 50, oxyd of iron oxyd of manga- 
nefe 28. - 
2. Magnefiata flexuofa, or irregular magnefia : reddilh 
or greyilh-white externally, (hining internally ; making a 
grey mark; breaking into indeterminate fragments, with 
the foliations- incurved. Found in Hercynia, and near 
Camfdorf and Scharbenberg in Sweden. 
3. Magnefiata fpatofa, or fparry magnefia: a little 
fliining internally, making a grey mark, breaking into 
rhomboidal fragments, w ith the foliations ftraight. There 
are many varieties of this fpecies, which are found in the 
mines of Hercynia and Saxony; diaphanous, fubopake, 
rarely opake; colour cinereous, reddilh or yellowiffi-white, 
ifabella, rofy, flefh-colour or brownilh-red, yellowilli or 
blackifii brown, with fometimes feveral of the colours 
blended together, and often with a metallic lullre; differs 
from the Ferrutn fpatofum by the fmaller proportion of 
iron and oxyd of manganefe it contains. 
4. Magnefiata nativa, magnefite or native magnefia. 
It was long denied that native magnefia exifted in a Hate 
of perfeft purity : it was difeovered, however, in that 
ilate in New Jerl'ey in the year 1810. It fomewhat referu¬ 
bles the talc of Mount St. Gothard, but with more of the 
fparry lullre which characterizes apophyllite : it confilts 
wholly of magnefia, combined only with the water of 
cryllaHization. Colour yellowiffi-grey, ordirty-yellowifli 
white; bluifli-white before it has been much expofed 
to the air. It is always found mafiive, generally in 
rounded pieces, fometimes of the fize of a man’s head, arid 
of earthy afpeCt ; thefe pieces are fometimes found with 
figures, but not with rounded cells. Its hardnefs varia¬ 
ble ; in its more compact ftate it fcratches calcareous fpar, 
but is fcratched by fluor fpar. It is alfo found in a friable 
ftate, when it foils the fingers. FraCture flat conchoidal, 
approaching to even ; dull. Its tenacity is inconfidefable. 
Fragments indeterminately angular, more or lefs ffiarp- 
edged. It is generally perfeCtly-opaque ; fometimes very 
thin fragments are tranflucent at the edges. It is fcarcely 
at all unCtuous to the feel; it adheres to the tongue. 
Specific gravity, when penetrated by water, 2-881 ; when 
not thoroughly faturated with it, 2-456. It is perfectly 
infufible before the blowpipe. In a llrong heat it con¬ 
tracts, and acquires a fufticient degree of hardnefs to 
fcratch glafs. It forms an excellent porcelain with filex. 
It is highly probable that this earth, like lime and 
barytes, is a compound, a peculiar metal united with 
oxygen, between which the affinity may be fo great as 
not to be obtained but with great difficulty. It is, there¬ 
fore, to future experiments that we mull look for the final 
eftablilhment of the compound nature of this earth. 
Dr. Black obferved, that magnefia was precipitated from 
acids by volatile alkali; that,, when uncalcined, it preci¬ 
pitated calcareous earths from acids ; but when calcined, 
or in any. other way deprived of its gas, it did not preci¬ 
10 7 
pitate thefe earths ; when uncalcined or united with gas, 
it precipitated lime from lime-water; which (hows that 
the calcareous earth had a (Ironger power to combine with 
the gas than the magnefia had, fince the former earth took 
this gas from the latter; by which means the former was 
rendered mild and infoluble in water, and therefore was 
precipitated. This property fuggefted to Dr. Allton 3 
method of procuring fvveet water at fea, by adding mag¬ 
nefia to water, the putrefaction of which has been pre¬ 
vented by the previous addition of quick-lime. M. Mon- 
net obferves, that magnefia, combined with fulphur, ge¬ 
nerally invelopes ftrata of coal. 
The directions for preparing it, given in the tall Lon¬ 
don Pharmacopoeia, are as follows : Take of fulphat of 
magnefia, (for the preparation of which, fee Chemistry, 
vol. iv. p. 231.) fubcarbonat of potafs, of each a pound, 
and water three gallons. Dill'olve the fubcarbonat'of po¬ 
tafs in three pints of the water, and drain ; difl'olve alfo 
the fulphat of magnefia feparately in five pints of the wa¬ 
ter, and drain ; then add the red of the water to the lat¬ 
ter folution ; apply heat, and, when it boils, pour in the 
former folution, (lining them well together; next drain 
through a linen cloth ; laltly, waffi the powder repeatedly 
with boiling water, and dry it upon bibulous paper in a 
heat of 200 0 . The double decompofition of the falts ufed 
in this procefs yields carbonat of magnefia and fulphat of 
potafs, the fird of which it is the object to collefl as free 
as pollible from the lad. Hence, as the newly-formed 
fulphat of potafs requires a large proportion of water for 
its lolution, fuch a proportion is directed in the fird in¬ 
dance, and it is afterwards well waffied with more. If 
water be impregnated with carbonic acid gas, it will di’f- 
folve carbonat of magnefia; and hence the liquor is made 
to boil, for the purpofe of detaching it. If the two dilu¬ 
tions be mixed cold, and the precipitate left for fome days 
upon the filter without artificial drying, many large and 
perfeCl crydals of carbonat of magnefia will be formed in 
it. The fubfequent heat, by which the powder is dried, 
(liquid not be great enough to detach any of the carbonic 
acid. The prefent procefs will yield a pure and elegant 
preparation ; its form is that of a white powder, ealily 
friable, and, according to Fourcroy, if the bafe be fully 
faturated with carbonic acid, as in the cryftals, (for in its 
ordinary form it is a fubcarbonat,) 100 parts contain of 
carbonic acid 50, of magnefia 25, of water 25 ; and if not lb 
faturated, but in its (late of fub-falt, of carbonic acid 48, 
of magnefia 40, and of water 12. In commerce, the mu- 
riat of magnefia contained in the refiduary liquor, after the 
cryilallization of inuriat of foda from fea-water, is decom- 
pofed by a fimilar procefs, and yields a large proportion of 
the ordinary magnefia of the markets. The dofe of car¬ 
bonat of magnefia to adults is from 3 JC, to 3 ii, and of 
fulphat from 5 i to §i. See Carbonat of Magnefia, vol. iv, 
p. 261. 
Mr. Henry, an ingenious apothecary at Manchefter, has 
communicated the following procefs for making the rnng- 
nefia. Difl'olve any quantity of fal catharticus amarus in 
its own weight of water; filter, and add to it by degrees 
a filtered folution of pearl-aflies in an equal quantity of 
water, flirring them gently, until the mixed liquors have 
acquired the appearance of a complete coaguium ; then 
delilt from adding any more of the alkaline lixivium, and 
immediately throw the mixture into a large vefiel of boil¬ 
ing water; keep it boiling fora quarter of an hour, then 
take it out, and put it into glazed"earthen veflels ; as foon 
as the powder has l'ubfided, and before the w-ater is quite 
cold, pour it off, and add a frelh quantity of boiling wa¬ 
ter; repeat thefe ablutions with hot water feveral times, 
till the liquor has entirely loft its (aline tafte ; then let it 
be fo agitated as- to fufpend the finer parts of-the powder, 
in which ftate decant it into other veffels, and, bavino- 
leparated the water from the magnefia by inclination, put 
it on large chalk-ltones, till a confiderable part of the hu¬ 
midity is abforbed ; then wrap it up in (beets of white 
paper, and dry it before the. fire. Pour hot water upon 
