It is well to bear in mind that the number of these _lusus naturæ_ seemed very much larger in the eyes of writers of a few centuries ago than to us to-day, for the numerous petrifactions, showing a great variety of animal and vegetable forms, were for a long period included in the same category with the stones bearing curiously deceptive markings or veinings. Much ingenuity was expended by early observers in the attempt to explain the cause of these phenomena. The learned Jesuit, Athanasius Kircher, for example, after having proved experimentally that designs treated with certain chemical agents could be made to impress figures upon stones, took refuge in the strange hypothesis that pictures made on wood or some soft material by primitive miners had been left in the mine and with the lapse of time had slipped down into crevices in the rock, and, becoming tightly wedged in, had impressed the design on the contact-rock; or else he suggested that the original material on which the design had been made might in process of time have, by some unknown means, been converted into marble.[87] As a striking example of a picture of this class, Kircher notes and figures an image naturally designed on a stone slab in St. Peter’s in Rome and bearing a remarkable likeness to the Blessed Virgin of Loreto.[88] The electric or magnetic gems, tourmaline, amber, and loadstone, possess not only great scientific interest, but demonstrate the fact that a certain energy really does proceed from some of these fair, ornamental objects, an energy that produces a positive action from without upon the human body. This may well serve to make us less resolutely sceptical as to the possible presence in gem-stones of some other forms of emanation not as yet susceptible of scientific determination.