The disintegration of a substance is much accelerated and extended by
the addition of other materials; hence the pharmaceutical aphorism of
Gaubius, “_Celerior atque facilior succedat composita, quam simplex
pulverisatio._” Thus several refractory vegetable bodies, as _myrrh_,
_gamboge_, &c. are easily reduced by triturating them with sugar or a
hard gum; and some gum resins, as _assafœtida_ or _scammony_, by the
addition of a few drops of almond oil. Upon the same principle the
Pharmacopœia directs the trituration of aloes with clean white sand, in
the process for preparing _Vinum Aloes_, to facilitate the pulverization
and to prevent the particles of aloes, when moistened by the liquid,
from running together into masses; some dispensers very judiciously
adopt the same mechanical expedient in making a tincture of myrrh; so
again, in ordering a watery infusion of opium, it will be judicious to
advise the previous trituration of the opium with some hard and
insoluble substance, as directed in the _Pulvis Cornu Usti cum Opio_,
otherwise its particles will adhere with tenacity, and the water be
accordingly unable to exert a solvent operation upon its substance.[295]
It is equally evident that in the construction of compound medicinal
powders, the addition of an inert ingredient, which the mere chemist
might condemn and discard as useless, not unfrequently acts a very
important part in the combination, owing to its effects in dividing and
comminuting the mere active constituents: the _sulphate of potass_ in
Dover’s powder acts merely in dividing and mixing more intimately the
particles of opium and ipecacuan: the _phosphate of lime_ appears to act
in the same mechanical manner in the Antimonial Powder; so again, in the
_Pulvis Contrajervæ compositus_, the prepared oyster shells may be a
necessary ingredient: in the _Pulvis Jalapæ compositus_ of the Edinburgh
College, the cream of tartar greatly increases the activity of the
jalap, by breaking down its substance and dividing its particles; and
Van Swieten observes that the operation of this resinous purgative is
improved by bruising it with sugar, and adding some aromatic. The old
combination of _Pulvis Helvetii_ consisted of alum and dragon’s blood,
and there can be no doubt but that the effect of this latter ingredient,
which has been often ridiculed, was to retard the solution of alum in
the stomach, in consequence of which the preparation was likely to
produce less inconvenience, and could therefore be administered in
larger doses; the Edinburgh college has substituted gum Kino in their
_Pulvis Aluminis compositus_, which may have the same effect in
modifying the solubility of the alum.
In rubbing together different substances, it is necessary to remember
that there are many saline bodies, which in the dry state become moist
and even liquid, by triture with each other, and that, under such
circumstances, they are susceptible of mutual decomposition. This change
is effected by the action of water, derived from the following sources.
1. _From the water of crystallization._ This always operates when the
proportion contained in the original ingredients is greater than that
which the products can dispose of; that is to say, whenever the capacity
of the new compound for water is less than that of the original
ingredients. By previously driving off this water by heat, we shall of
course avoid such a source of solution, and no liquefaction can ensue.
Thus, if recently burnt quick-lime be triturated with calomel, the
resulting mixture will be white, shewing that no decomposition can have
arisen, but add a few drops of water, and it instantly assumes a dark
aspect. If _crystallized_ sulphate of copper be triturated with Acetate
of lead, the resulting mixture will assume a fine green colour, but if
the sulphate of copper be previously heated, and its water of
crystallization driven off, no change of colour will be produced; if,
for Acetate of lead, we substitute muriate of lime, and the sulphate of
copper be _crystallized_, we shall obtain a result of a yellow colour,
but if the sulphate of copper be _anhydrous_, the product will be
colourless, becoming however instantly yellow, like the former, on the
addition of a drop of water; and on a further addition of this fluid,
the yellow product in both instances will be rendered blue; which proves
that a chemical decomposition has taken place, and a muriate of copper
resulted; for this salt is rendered _yellow_ by a small, and _blue_ by a
larger proportion of water. The _Cuprum Ammoniatum_ presents another
illustration, for the ingredients, when rubbed together, become
extremely moist, and undergo a chemical decomposition. Certain resinous
bodies also, as _myrrh_, become liquid by triture with alkaline salts,
in which case the resin and alkali form a soluble compound, which the
water of crystallization, thus set at liberty, instantly dissolves.
2. _From aqueous vapour in the atmosphere._ The water of the atmosphere
does not act upon these occasions, unless it be first attracted and
absorbed by one of the triturated bodies; e. g. if Acetate of lead and
recently burnt alum be triturated together, no change will be produced;
but, if the burnt alum be previously exposed for a short time to the
atmosphere, these bodies will, in that case, become liquid.
The physician, without this chemical knowledge, will be often betrayed
into the most ridiculous blunders, an instance of which very lately came
to my knowledge in a prescription for the relief of cardialgia and
constipation, in the case of dyspepsia; it directed _sulphate of soda_
and _carbonate of potass_, in the form of a powder, but the _fiat_ of
the physician, upon this occasion, only served to excite the ridicule of
the dispenser, who soon discovered that the ingredients in his mortar
dissolved into liquid.
During the exhibition of powders containing insoluble matter, it is
always important to maintain a regularity in the alvine excretions, or
an accumulation may take place attended with very distressing symptoms.
Dr. Fothergill relates a case of this kind which succeeded the use of
powdered bark; and Mr. E. Brande has communicated a similar instance of
mechanical obstruction, produced by the habitual use of magnesia. I
could also add, if it were necessary, some striking facts of a similar
tendency, which occurred from eating bread that had been adulterated
with pulverized _felspar_. The precaution seems more particularly
necessary in the case of children, whose bowels are very impatient of
extraneous and insoluble contents.[296] The dose of a powder ought not
to exceed ʒj; and, when taken, should be diffused in water, wine, or any
other convenient liquid; resinous and metallic powders require a thick
and consistent vehicle, as syrup or honey, since they subside from those
which are more fluid.