In his commentary on Theophrastus, Sir John Hill touches upon the question of the medicinal virtues of precious stones. His researches regarding the causes and conditions determining color in stones, led him to the conjecture that the active principle, if it really existed, was to be sought in the coloring matter. As the opinion of a very clever student in his day, his words will bear quotation:[224] The greatest part of these [medicinal virtues] cannot but be seen at first view to be altogether imaginary; and as to the virtues of the Gems in general, it is now the reigning Opinion, that they are nearly all so, their greatest Friends allowing them no other than those of the common alkaline Absorbents. However, whether the metalline Particles, to which they owe their Colours, are, in either Quantity or Quality, in Condition to have any effect in the Body, is a Matter worthy of a strict and regular Tryal; and that would at once decide the Question between us and the Antients, and shew whether we have been too rash, or they too superstitious. The so-called “doctrine of signatures” treated of the marks set by nature upon certain objects to denote their usefulness in the cure of diseases affecting different parts of the body, or their power to neutralize the effects of the bites of certain animals or reptiles. Of this theory Martius says that the “signatures” are not to be sought in a fanciful resemblance to the form of the objects with the diseased parts of the human body, but rather in the color, odor, taste, composition, etc., of the objects.[225]