longer past experience than had the astrologer to convince him of the futility of identifying mind and matter. The only other science which aroused popular interest was alchemy. The basis of this subject was the postulate that all matter was composed of the four elements combined in various proportions. By the use of chemical reagents, the combinations of these elements could be altered and a given substance be thus changed into another. Alchemy is generally associated with the attempt to transmute metals and, in particular, to change lead into gold, because the natural cupidity of their patrons made it advisable for the alchemists to hold that prospect before them in order to obtain a livelihood. In principle, alchemy is but little different from modern chemistry. Our most recent theory still holds that the difference between lead and gold is due only to the numerical relations of a single element, the electron. The difference between chemistry and alchemy does not lie in either their fundamental hypotheses or their methods, but in our vastly superior technique of experimentation and accumulated knowledge. The nature of the modern electronic atom is essentially as fictitious as was the nature of the mediæval elements. Besides exciting the hope of wealth, alchemy was important as an aid to health and longevity. Since health required that the four elements of the body should be preserved in their proper balance, illness, and even death, were but the temporary or permanent loss of their right proportions. Alchemists, convinced of this truth, were led to seek for a sovereign substance, the philosopher’s stone, which would have the power to restore this disturbed balance and give to its fortunate owner permanent health and life. It is a mistake to suppose that the Church oppressed these sciences--many of its most orthodox fathers and saints eagerly studied them. The abuses due to the rampant charlatanry of many of their practitioners were repressed, but their serious doctrines were fused into the religion of the time much as, in our day, the clergy have tried to harmonise Christianity and biological evolution. In fact, as we shall see, the most determined opposition which the new Copernican theory of the solar system had to overcome was the fusion of Christian dogma with Aristotelian astrology and alchemy.