dents of chemistry down to the time of Lavoisier, and the aims and practice of alchemy, which for many centuries was the only phase of chemistry studied, were wholly in harmony with this conception. If the metals were all manifestations of the same underlying essence, and differed only in the accidents of external qualities, it was reasonable to suppose that these accidents might be changed. The alchemists were often intelligent men, and knew as well as ourselves that "all is not gold that glitters"; but the resemblances to the precious metals which they sometimes obtained by their empirical methods were sufficient to stimulate effort. They also clearly saw that the value of the prize they sought would vanish in their keeping the moment the secret became known; but this only led them, as it does so many manufacturers of the present day, to invest their processes with all possible mystery, to conceal known facts beneath non-essentials, and to adopt a conventional and highly figurative language for communicating with each other, so that, even with our knowledge of chemistry, the writings of the alchemists are for the most part an unintelligible jargon. Still, their hopes were based on what they regarded as sound philosophy; and, although their efforts were frequently exposed to ridicule on the ground of ill success, no convincing objections were ever raised to the philosophy by which they were guided. That the aims of the alchemists must have appeared reasonable to thinking men is shown by the fact that, even at a late period in the history of this apparent delusion. Sir Isaac Newton, whose scientific sobriety can not be questioned, devoted a great deal of time to experiments on the transmutation of the metals.
During the two thousand years through which the doctrine of a few elementary principles of nature prevailed, the precise form which the elements assumed naturally varied with the general point of view of the students at the time, although for the most part philosophical writers adhered to the statement of Aristotle. By many of the alchemists mercury, sulphur, and salt were regarded as fundamental principles, because the crude materials under these names played such an important part in the hermetic art. Here, however, it was not these crude materials which were regarded as the elements of matter, but sublimated forms of these substances, known as the mercury and sulphur of the philosophers; and for a long time the conceit was cherished that, if once the elemental mercury and sulphur could be isolated, all metals, and, of course, gold and silver among the number, could be manufactured by mixing these elements in the right proportions. Later, when chemistry assumed a pharmaceutical character, the elements were often said to be water, spirit, oil, salt, and earth, of which the first three were regarded as active and the last two as