Although MAGNESIA ALBA is a medicine which has been in general use for
many years, yet the proper mode of making it is very little known. Our
_pharmacopæia_ affords us no information about it; and the _formula_
which is given by the Edinburgh College, as well as that with which
the ingenious Doctor Black[b] has favoured us, is deficient in several
circumstances. Hence the preparation of pure Magnesia has been confined
to very few persons, who have preferred the emolument they have
received by keeping their method secret, to the more diffusive utility
of which a publication of it would have been productive.[c] I therefore
beg leave to lay before the public a process for the preparation of
Magnesia, by which it will be in the power of every Apothecary to make
it himself, in all respects equal to that which is sold by those who
conceal their method.
[b] Essays and Observations Physical and Literary, Vol. II.
[c] Mr. Glass, a few years since, published an Essay on Magnesia Alba,
in which all the information he affords us relative to the preparation
is what we knew before, viz. that it is prepared from Epsom salts and
pot ashes; and has related a number of difficulties which occur in the
process, at the same time carefully, I had almost said meanly, avoiding
giving the least instruction which might teach us how to shun them,
though he has given a long detail of the many tragical consequences
which may attend the use of Magnesia prepared under such disadvantages.
I am the more strongly induced to make this communication, because
the Magnesia which is generally to be found in the shops, is either
extremely coarse and ill prepared, or, which is still worse,
sophisticated with calcareous substances, differing greatly from true
Magnesia. I have been assured by some Physicians, that they have met
with it mixed with chalk, and even with lime, and I have sometimes seen
it so adulterated: a fraud of very dangerous tendency, as this powder
is so frequently administered to very young infants, and to adults of
tender bowels and costive habits.
This medicine was originally prepared abroad, from the liquor called
the _mother of nitre_, which is composed of a light earth united with
an acid; and these being separated, either by a strong fire, or by the
addition of an alkali, the powder was washed in water, and obtained the
name of Magnesia Alba. Hoffman afterwards prepared it from the bittern
remaining after the crystallization of sea salt, which he found to be
similar to the mother of nitre. And the factitious Epsom salt being
prepared from this bittern, and evidently composed of Magnesia and the
vitriolic acid, Dr. Black, who has favoured the world with a number of
very valuable experiments on this subject, made use of this salt with
success for the same purpose.
Happening some years ago to live in the neighbourhood of a gentleman
who has long been celebrated as the preparer of the most genuine
Magnesia, and never having been able myself to make Magnesia comparable
to his, by the commonly known methods, I was desirous of gaining some
intelligence as to his process, and was at last so fortunate as to
obtain some useful hints.
I availed myself of these, and after repeated trials, produced Magnesia
equally pure, white, tasteless, light and impalpable with that of Mr.
Glass; nay sometimes that of my own preparing has been superiour to
his, and in one respect has generally the advantage of it, namely,
that mine is not so stiff when dried, and may be reduced to the
finest powder by simple pressure; whereas _his_ requires some degree
of trituration to break the lumps effectually; which I imagine may be
owing to his pursuing Dr. Black's method of drying it, by straining and
_pressing_ out the water through a cloth.
The following is the manner of preparing it, which I have found
successful.
Dissolve any quantity of _sal catharticus amarus_, commonly called
Epsom salts, in its own weight of water; filter the liquor, and add to
it by degrees a filtrated solution of pearl ashes in an equal quantity
of water, stirring them gently until the mixed liquors have acquired
the appearance of a complete coagulum: then cease adding any more of
the alkaline lixivium; and, having diluted the precipitate, and mixed
it intimately with a small quantity of hot water, immediately throw
the mixture into a large vessel of boiling water. Keep it boiling for
a quarter of an hour, then take it out, and put it into glazed earthen
vessels. As soon as the powder has subsided, and before the water be
quite cold, pour it off, and add a fresh quantity of boiling water:
repeat these ablutions with several parcels of hot water, till the
liquor has entirely lost its saline taste. Then let it be so agitated
as to suspend the finer parts of the powder; in which state decant it
into other vessels, and having separated the water from the Magnesia
by inclination, put it on large chalk stones, till a considerable part
of the humidity be absorbed. Then wrap it up in sheets of white paper,
and dry it before the fire. Pour hot water on the remaining powder,
stir it, decant it in its turbid state, and separate the Magnesia from
the water as before. By these means, the whole, or most of it, will be
reduced to an equal degree of fineness.
The separation of the Magnesia will be promoted by heating the saline
lixivia before they are mixed; and the larger the quantity of water
into which the precipitated powder is cast, the more speedily and
perfectly will the vitriolated tartar, which is formed by the alkali
of the _sal catharticus_, be washed off. Dr. Black directs that three
or four times the quantity of water, to that of the solutions, should
be added; but this I have found greatly insufficient. The neutral
salt should be washed off as quickly as possible; otherwise, as he
justly observes, by allowing the mixture to stand for some time,
the powder concretes into minute grains, which when viewed with a
microscope, appear to be assemblages of needles diverging from a
point. These concretions cannot be redissolved by any washing, however
long continued. His intention, in boiling the mixture, is much better
answered, by adding it to the water when in a state of ebullition; and
once boiling in this manner is more effectual than a dozen washings in
hot water.
Much depends on the purity of the water used in the process. If it be
hard pump water, the selenites with which it is impregnated will be
decompounded, and the calcareous earth be deposited, after boiling;
which mixing with the Magnesia will render it impure, gritty and
discoloured. Rain water collected free from impurities, or clear
river water, are most eligible; but if the situation of the operator
does not permit him to procure these in a proper state, he should
either use distilled water, which has been kept till the empyreuma is
gone off, or at least such pump water as is free from any calcareous
or saline impregnation. When poured on the Magnesia, it should be
strained through a thick linen cloth, so as to intercept any accidental
impurities which it may acquire in heating.
The drying should be performed with expedition. To this end, the chalk
stones should be exposed to a moderate degree of heat; and when
they have been employed two or three times, should be dried before a
fresh quantity of the Magnesia is put on them. Cleanliness should be
particularly attended to through the whole process; and the vessels
ought to be carefully covered, that no dust may enter.
We may safely make use of a large copper brewing-pan, to boil the
Magnesia in; for as the acid is perfectly neutralized, there can be no
danger of its quitting the alkali, to which it has a greater affinity
than to the metal; and copper does not readily dissolve, even in acids,
when boiling hot; nor have I ever observed the least corrosion, though
I have frequently used such vessels for this purpose.