As sources for ascertaining the medicinal virtues of drugs were so soon exhausted, the systematizer of the materia medica bethought himself of others, which he deemed of a more certain character. He sought for them in the drugs themselves; he imagined he would find in them hints for his guidance.
He did not observe, however, that their sensible external signs are often very deceptive, as deceptive as the physiognomy is in indicating the thoughts of the heart.
Lurid-coloured plants are by no means always poisonous; and on the other hand, an agreeable colour of the flowers is far from being any proof of the harmlessness of the plant. The special qualities of drugs, which may be ascertained by the smell and the taste, will not allow us to form any trustworthy conclusions respecting untried substances.
I am far from denying utility to both these senses in corroborating the probable properties of drugs which have been ascertained in other ways, but I would counsel, on the other hand, great caution to those who would form their judgment from them alone.
If the bitter principle strengthens the stomach, why does squill weaken it? If bitter aromatic substances are heating, why does marsh rosemary diminish the vital temperature in such a marked manner? If those plants only are astringent that make ink with sulphate of iron, how is it that the highly astringent principle in quinces, medlars, andc., cannot furnish ink?
If the astringent taste gives evidence of a strengthening substance, why does sulphate of zinc excite vomiting? If the acids are antiseptic, why does arsenious acid produce such rapid putrefaction in the body of one poisoned by it?
Is the sweet taste of sugar of lead a sign of its nutritive properties? If the volatile oils, and everything that tastes fiery on the tongue, are heating for the blood, why are either, camphor, cajeput oil, oil of peppermint, and the volatile oil of bitter almonds and cherrylaurel, the very reverse?
If we are to expect a disagreeable odour in poisonous plants, how is it so inconsiderable in monkshood, deadly nightshade, and foxglove? Why so imperceptible in nux vomica and gamboge?
If we are to look for a disagreeable taste in poisonous plants, why is the most deadly juice of the root of jatropha manihot merely sweetish, and not the least acrid?
If the expressed fatty oils are often emollient, does it follow that they are all so, even the inflammatory oil expressed from the seeds of the jatropha curcas? Are substances which have little or no smell or taste destitute of medicinal powers?
How is that ipecacuan, tartar emetic, the poison of vipers, nitrogen, and lopez-root, are not so? Who would use bryony-root as an article of diet, on the ground that it contains much starch?