Magnesia Vitriolata. Sal catharticum amarum.
_Bitter purging Salt._ _Epsom Salt._
QUALITIES. _Form_, small needle-like crystals. _Taste_, bitter and
nauseous; when pure, it effloresces. CHEMICAL COMPOSITION. In its
crystallized state, it may be considered as composed of 1 proportional
of dry sulphate (Magnesia 18·5, and sulphuric acid 37·5) and 7
proportionals of water. SOLUBILITY. f℥j of water dissolves ℥j, and the
solution measures fʒxj¼; it is insoluble in alcohol. INCOMPATIBLE
SUBSTANCES. _Muriates of ammonia, baryta, and lime_; _nitrate of
silver_; _sub-acetate_, and _acetate of lead_. _The fixed alkalies_ and
_their carbonates_, precipitate from it magnesia and its carbonate.
_Phosphate of soda_ occasions no immediate precipitate, unless ammonia
be present, in which case the triple _ammoniaco-magnesian phosphate_
will be produced. The addition of ammonia, which in the form of
_Spiritus ammoniæ aromat_. is not unfrequently prescribed in conjunction
with a solution of this sulphate, forms also a triple salt, and a
portion of magnesia is precipitated: whenever therefore this ammoniacal
stimulant is ordered with a purgative salt, the scientific physician
will prefer a solution of the sulphate of soda. FORMS OF EXHIBITION.
Dissolved in the _Infusum Rosæ_, or in a suitable quantity of beef tea,
gruel, or any aqueous vehicle, its cathartic powers are increased by
dilution, as well as by the addition of a little common salt; _magnesia_
renders the taste of its solution less nauseous; and tartarized antimony
quickens its operation. DOSE, ℥ss to ℥ij, taken either at once, or in
divided doses. _Form. 37, 69, 72, 76._ OFFICINAL PREP. _Enema
Catharticum._ _Enema Fœtid._ D. ADULTERATIONS. _Sulphate of Soda_ is
often substituted for this salt, which it may be made to resemble by
stirring it briskly at the moment when it is about to crystallize; the
fraud may be detected by a precipitation not ensuing on adding carbonate
of potass; if only a part of the salt be sulphate of soda, the degree of
sophistication can be learnt by the quantity of the precipitate formed;
100 parts of sulphate of magnesia, if pure, will yield between 30 and 40
of the dry carbonate. Epsom salt, as it commonly occurs, contains
_muriate of magnesia_, which disposes it to deliquesce, but lately this
salt has appeared in the market in a state of great purity and beauty;
the mode of purification is founded upon the well known chemical law,
that _a saturated solution of one salt is still capable of dissolving
another_; in the present instance, therefore, the impure crystals are
washed in a saturated solution of the same sulphate, which, although
unable to act upon its kindred salt, can dissolve with facility the
muriate, and any other saline contamination. I confess, however, that I
am induced to regard this process as rather chemically ingenious than as
medicinally useful, for the usual saline impurities of Epsom salt are
not only harmless, but capable of increasing its purgative powers; the
_double refined_ sulphate is certainly less efficient as a cathartic.
The presence of the _muriate_ may be at once detected by dropping upon
the suspected sample some sulphuric acid, by which the disengagement of
muriatic acid vapour will be produced. Since the publication of the
fourth edition of the present work, I have received samples of the
sulphate of Magnesia, prepared by Mr. West of Lymington, and I can
confidently recommend the article to the profession; he contrives to
obtain them in large and beautiful crystals, which cannot be so easily
mistaken for those of oxalic acid, a circumstance of no small importance
to the drug vender; the form of these crystals is that of a square
prism, having its edges replaced, and commonly terminated by a pyramid
of four planes; the only cleavage is parallel to one of the diagonals of
the prism. The numerous accidents which so frequently occur from
mistaking Oxalic acid for Epsom Salts have given rise to many
suggestions for obtaining an easy and popular test which may at once
distinguish these bodies; it is evident that no test can be so simple as
that afforded by the taste; but if such accidents are in future to be
prevented, it must be done by imparting to the acid some external
character by which it may be at once recognised; if a test were even
discovered a hundred times more sensible than any which we possess, what
would it avail?