This may be termed an error in the _Mechanism_ of the Prescription, and has been generally regarded as being more inconvenient than dangerous, more fatal to the credit of the Prescriber than to the case of the Patient: the observations however which are offered in this work, especially under the article _Pilulæ_, must satisfy the practitioner that this error is more mischievous in its effects than has been usually supposed; it is so palpable and self-evident in its nature, that it will be unnecessary to illustrate it by more than one or two examples. _Calomel_, for instance, has been ordered in an aqueous vehicle, and certain _resinous tinctures_ have been directed in draughts, without the necessary intervention of mucilage; so again, an intermixture of substances has been formerly ordered in powder that possess the perverse property of becoming liquid by triture (see _Pulveres_), and bodies have been prescribed in the form of pills, whose consistence[277] renders it impossible that they should preserve the globular form; or else they have been so hard and insoluble, that they might be fired through a deal board.[278] In the London Pharmacopœia of 1809, an error of this kind unfortunately passed without correction with regard to the Formula for preparing the _Syrup of Senna_. II.—_Substances are added together which mutually decompose each other, whence their original virtues are changed, or destroyed._ This is a more serious, but not a less frequent source of error; it has been already shewn in this Analysis (IV. B) that the judicious and scientific application of chemical science has furnished new and endless resources to the physician, by exalting the efficacy and correcting the acrimony of established remedies, or by combining inert substances so as to create new and powerful medicines. With equal truth and confidence it may be asserted, that the abuse of these means not only destroys the virtues of the most valuable articles in the _Materia Medica_, but that the mildest remedy may be thus converted into an instrument of torture, and even of death. In a lecture delivered at Apothecaries’ Hall, Mr. Brande stated that he had seen a prescription in which the _blue_, or mercurial pill, was ordered in conjunction with nitric acid, and that the patient was brought to “death’s door” from the formation of _nitrate of mercury_ in his stomach! I have myself lately seen a _Recipe_, professing to afford a preparation similar to the “_Black Drop_,” and which directed a mixture of a _Tincture of Opium_, made with rectified spirit, with _Nitric Acid_; in this case, it may be very safely inferred that the author was not only ignorant of the chemical habitudes of these bodies, but that he never performed the experiment in question, or he would have learnt from dire experience, that in consequence of the rapid evolution of _nitric ether_, the contents of the phial will explode with violence, to the imminent hazard of the operator’s eye-sight. During the