Since the drawing up of the paper which has been the subject of the
last chapter, some observations have occurred, which are either so
immediately connected with, or at least deduced from it, that it may
not perhaps seem inexpedient to introduce them in this place.
I was very much surprised to observe, in the Lectures lately published,
_as delivered_ by the very learned and ingenious Dr. Cullen, that
Magnesia is there said to be no more purgative than any other absorbent
earth. The sentence is this; (speaking of other absorbents) "Magnesia
alba should have been added to this set. It has had a considerable
reputation as an absorbent, and when neutralized, as a purgative; but I
find it is not more absorbent than any of the rest, nor more purgative
in less quantity, as chalk or crab's eyes given in the same quantity,
viz. dramij, will have the same effect. Therefore it may be neglected."
Surely this must have been an error of the person who wrote down the
lectures, and have escaped the notice of the ingenious editors; who,
from their skill in chemistry, must know that pure Magnesia differs
from every calcareous or testaceous earth with which we are hitherto
acquainted. These earths are nearly insoluble in the vitriolic acid,
and what part does unite with it, forms a selenitical salt, the most
difficult of solution of all others, and of an astringent nature:
whereas the Magnesia[d] united with the same acid, produces what is
commonly called Epsom salt, easy of solution, and purgative to the
bowels. The former, with the nitrous acid constitutes a calcareous
nitre, incapable of crystallization; with the marine acid a calcareous
muriatic salt; and when dissolved in vinegar, the mixture spontaneously
dries up into a friable sub-astringent salt: whereas Magnesia, with
all these acids, forms _purging salts_; that with the nitrous acid,
yellow, capable of being reduced into crystals retaining their form in
a dry air, but melting in a moist one: with the muriatic acid, a salt
is produced which does not crystallize, and easily melts when exposed
to the air: with distilled vinegar, a saline uncrystallizable mass is
formed, resembling glue both in colour and consistence while warm, but
becoming brittle when cold. Dr. Black says, that two drachms of this
salt purged a middle aged man four times; and half an ounce of the same
gave a woman of a strong constitution no less than ten stools.[e]
[d] Essays Physical and Literary, Vol. II. p. 164.
[e] Ibid. p. 64.
Besides, where an acid prevails, much smaller doses than two drachms of
Magnesia prove purgative; and it seldom happens that even that dose of
the other absorbents[f] will produce the same effect. Nor am I singular
in my opinion, when I declare my doubt whether Magnesia be not of
itself in some degree purgative, independent of its junction with any
acid whatsoever. It appears to be an earth _sui generis_. That of alum
resembles it in some respects, yet differs from it essentially, when
combined with the vitriolic acid: the alum is strongly astringent and
antiseptic, the Epsom salt purgative and septic.
[f] Hoffman, having attributed the purgative quality of Magnesia to
its forming a bitter cathartic salt with the acid it meets with in
the stomach and bowels, adds, "At vero in contrarium quoddam dubium
contra hanc sententiam moveri posse intelligo, quum nempe alia terrea,
quæ prompte solvent et absorbent inhærescens primis viis acidum,
neutiquam effectum laxantem exserant. Sed his regerere licet, quod
interdum a pulveribus absorbentibus vel bezoardicis utique alvus
fluidior fiat, si multum acidi primam regionem incolet: vis tamen eorum
purgandi non tanta est, quanta magnesiæ, quia solutiones illorum cum
acidis liquoribus factæ non tam eminente salino acri, sed moderate
salso sapore imbutæ sunt, quam quidem ea, quæ ex magnesia et acidis
liquoribus conficitur. Atque adeo ex eo apparet, præter alcali terreum
aliud adhuc esse in magnesia principium, quod ad mixturam acidi in
materiem stimulantem et purgantem transeat."
Hoffman. Animadversiones et experimenta circa Magnesiam, &c.
Op. Tom. 4. p. 480.
I have very lately seen a paper signed by Doctor Cadogan and dated in
the year 1767, in which he complains grievously of the advertisers of
Magnesia, making use of his name without his consent, and has published
the process for making _his_ Magnesia. The doctor's intent in this
was doubtless benevolent, but his manner of preparing this powder is
unnecessarily expensive and wasteful. He directs only one pound of
lixivium tartari to five pounds of sal catharticus amarus, which is
greatly insufficient to precipitate all the Magnesia. And he insists
strongly on the superiority of the lixivium prepared from salt of
tartar, to that made of potashes, as if the chemical effects of one,
were different from the other. But, says the Doctor, potashes render
the Magnesia bitter. Surely the vitriolated tartar produced by a union
of one vegetable fixed alkali with the vitriolic acid, is equally
soluble in water with that prepared with any other, and if so, will be
as easily washed off from the Magnesia.
But behold a champion steps forth, and at one blow levels to the
ground the whole tribe of Magnesia makers, who have procured it from
the factitious Epsom salts. I confess I have not had the happiness to
peruse this ingenious gentleman's pamphlet on the subject, but I have
formed a very extraordinary opinion of his _candour_, _modesty_, _and
knowledge_, from the very curious paper which he distributes with his
Magnesia. Notwithstanding Doctor Black, and since him Mr. Glass and
several others, have procured _pure_ Magnesia from the factitious Epsom
salts, Mr. Dale Ingram, assures us, that he has made an improvement,
"which is by the learned esteemed one of the greatest acquisitions to
the materia medica." And wherein does this mighty discovery consist?
even that Magnesia prepared from the waters of Epsom, is superiour to
that prepared from the bitter purging salt; and he assures us that the
Magnesia sold by him is so prepared.
To the first assertion I shall only reply, that every person at all
conversant in chemistry knows that Magnesia earth is the same, from
whatever substance it can be separated in a pure state; that the
factitious Epsom salt yields it in as great a degree of purity as the
salt of the Epsom water, and that Dr. Alston assures us, the artificial
salt "by various and repeated experiments, made in France as well as in
Britain, is demonstrated to be every way as good as, yea to be the very
same with, the genuine made of the Epsom waters."[g]
[g] Alston's Materia Medica, Vol. I. p. 164.
As to the other declaration, it will be sufficient to observe that
one gallon of Epsom water contains only seven drachms of salt in a
dry season, and hardly six drachms in a wet one;[h] and that for this
salt to precipitate its Magnesia properly, it is necessary it should
be diluted with little more than its own weight of water.[i] Six
drachms of salt will yield two drachms of Magnesia. So that to procure
a pound of this powder Mr. Ingram must evaporate above sixty gallons
of the water, to between five and six pints, before he begins the
precipitation. Sure never did empiricism appear so thinly disguised!
[h] Ibid.
[i] Essays Physical and Literary, p. 163.
In the preceding chapter, the necessity of using water free from any
calcareous impregnation has been particularly insisted on, and I
have, on another occasion,[j] observed that great attention should
be given to the purity of the water used in the making of all the
saline preparations; and I may add in almost all the operations of
pharmacy. Dr. Percival, in his ingenious experiments on water, found
a quart of the Manchester pump water to contain upwards of sixty
grains of adventitious matter.[k] Suppose therefore, for instance,
that in making the extract from a pound of peruvian bark, it be
boiled only six times in the quantity of water directed by the London
Dispensatory, nine gallons will be consumed in the process; which
is a very moderate allowance, six coctions not being sufficient to
extract all the virtues of that drug. Dr. Percival boiled half an
ounce of bark twenty five times, in so many different pints of water,
the last of which had some impregnation, and the residuum gave a deep
colour, and considerable bitterness to rectified spirit of wine. If we
likewise suppose only one half of the foreign contents of such water
to be left by evaporation, then the quantity of calcareous and saline
matter, undesignedly mixed with the extract, will be two ounces and
two drachms, or nearly equal to the quantity of extract procured from
a pound of bark by pure water. Thus this important medicine becomes
grossly adulterated, without any such intention in the operator; and I
know it is the common practice to use pump water in making it.
[j] Vide Percival's Essays, 2d. Edit. p. 321.
[k] Ibid. p. 87.
I have particularly selected the Peruvian bark, as requiring a very
large quantity of water to extract the whole which it is capable of
yielding; but the proportion of water which I have here allowed, will
not be too great in obtaining extracts from most vegetable substances;
and how greatly not only the quantity, but the quality of the medicine
must be affected by the admixture of such a weight of insoluble
calcareous earth, is so obvious, that it is needless to expatiate on
it.