A very common error may be here noticed, which is that of prescribing a substance in such a form, as not to be acted upon with any effect by the solvent; as an example it may be stated, that in preparing an infusion of _Juniper Berries_, unless pains be taken by strong contusion to break the seed, it will contain but little power as a medicine. The unbruised seeds of Mustard were commended by Dr. Mead,[284] in Ascites, and by Bergius, in Intermittents; Dr. Cullen, however, has very properly observed that the seeds given in the above manner are never broken down or dissolved in the stomach, but pass away entire by stool, and cannot therefore occasion any beneficial result. It is unnecessary to multiply examples in proof of the numerous errors into which a physician must unavoidably fall, who presumes to compose prescriptions without a knowledge of the chemical habitudes of the different substances which he combines. The file of every apothecary would furnish a volume of instances, where the ingredients of the prescription are fighting together in the dark, or at least are so adverse to each other, as to constitute a most incongruous and chaotic mass. “Obstabat aliis aliud: quia corpore in uno Frigida pugnabant calidis, humentia siccis, Mollia cum duris, sine pondere, habentia pondus.” _Ovid. Metamorph._ lib. 1. 19. THE DOSES OF MEDICINAL SUBSTANCES are specific with respect to each, and can therefore be only learnt from experience; the young and eager practitioner, however, is too often betrayed into the error of supposing that the powers of a remedy always increase in an equal ratio with its dose, whereas THE DOSE ALONE VERY OFTEN DETERMINES ITS SPECIFIC ACTION. “_Medicines_,” says Linnæus, “_differ from poisons, not in their nature, but in their dose_,” which is but a paraphrase of the well known aphorism of Pliny, “_Ubi virus, ibi virtus_.”[285]—So that food, remedies, and poisons, may be said to branch into each other by indefinable gradations;—Five grains of _Camphor_ act as a mild sedative and slight diaphoretic, but twenty grains induce nausea, and act as a stimulant; so again, _Opium_, in too large doses, instead of promoting, prevents sleep, and rather stimulates the bowels than acts as a narcotic. Two ounces of any neutral salt are apt to be emetic, one ounce even of _Alum_ to be cathartic, and two drachms to be refrigerant; in like manner the preparations of _Antimony_ either vomit, purge, or sweat, according to the quantity exhibited.