2. Many substances are, in themselves, so untractable, that the addition of some matter foreign to the active ingredients, is absolutely essential for imparting convenience of form. It is generally considered that very little skill and judgment is required in the selection of such a substance, provided it can fulfil the _mechanical_ intention just alluded to—the fact however is, that _the medicinal power of the pill may be materially controlled, modified, or even subverted, by the mode in which it is formed into a mass_. Where the active element of a pill is likely to be improved by minute division, a gummy or resinous constituent may be usefully selected: under the history of Aloes, I have alluded to a popular pill, known by the name of the _dinner-pill_, in which case the _mastiche_ divides the particles of the aloes, and modifies the solubility of the mass. The _Pilulæ Opii_ of the former Pharmacopœia of London, consisted of equal proportions of opium and extract of liquorice, and the mass was so insoluble that its effects were extremely uncertain and precarious; in the present edition, soap has been very judiciously substituted; but in certain cases where we wish to protract the influence of opium, or that of any other active body, so as not to obtain its full effects at once, we may very advantageously modify its solubility by combining it with a gum resin or some substance which will have the effect of retarding its solution in the stomach. The _Pilulæ Styrace_ of the Dublin college, presents itself as an efficient example of this species of pharmaceutical address; see also _Form. 10, 11, 12_. I am well acquainted with many formulæ whose utility has been sanctioned by experience, and I have no hesitation in believing that their salutary mode of operation would receive a plausible explanation from this simple law of combination. Dr. Young has very justly stated in his Medical Literature,[297] that the _balsam of copaiba_ envelopes metallic salts, so as to lessen their activity; he says that the sub-carbonate of iron, made into pills with copaiba, was given for some weeks without any apparent effect; and that a few hours after the same quantity had been given, with gum only, the fæces were perfectly black. I do not know a more striking and instructive proof of the influence of a glutinous or viscid constituent, in wrapping up a metallic salt, and defending the stomach from its action, than is presented in the case published by the medical attendant Mr. Marshall, in consequence of the attempt of Eliz. Fenning to poison the family of Mr. Turner of Chancery-lane by arsenic, which she providentially administered in a heavy yeast dumpling. _Soap_ is very frequently used for the formation of pill-masses, and it is an excellent constituent for substances likely to be injured by meeting with an acid in the _primæ viæ_; many resinous bodies may also be reduced to a proper consistence