There have been very strange and incredible stories told and by the alchemists. Perhaps in some of them there may have been a specific form of mania, originating in the constant intension of the mind on an imaginary end, associated with an immense variety of means, all of them substances not familiar to men in general, and in forms strange and unlike those of ordinary nature. Sometimes, it seems as if the alchemists wrote like the Pythagoreans on music, imagining a metaphysical and inaudible music as the basis of the audible. It is clear that by sulphur they meant the solar rays or light, and by mercury the principle of ponderability, so that their theory was the same with that of the Heraclitic physics, or the modern German Naturphilosophie, which deduces all things from light and gravitation, each being bipolar; gravitation = north and south, or attraction and repulsion: light = east and west, or contraction and dilation; and gold being the tetrad, or interpenetration of both, as water was the dyad of light, and iron the dyad of gravitation.
It is, probably, unjust to accuse the alchemists generally of dabbling with attempts at magic in the common sense of the term. The supposed excercise of magical power always involved some moral guilt, directly or indirectly, as in the stealing a piece of meat to lay on warts, touching humours with the hand of an executed person, &c. Rites of this sort and other practices of sorcery have always been regarded with trembling abhorrence by all nations, even the most ignorant, as in the Africans, the Hudson's Bay people and others. The alchemists were, no doubt, often considered as dealers in art magic, and many of them were not unwilling that such a belief should be prevalent; and the more earnest among them evidently looked at their association of substances, fumigations, and other chemical operations as merely ceremonial, and seem, therefore, to have had a deeper meaning, that of evoking a latent power. It would be profitable to make a collection of all the cases of cures by magical charms and incantations; much useful information might, probably, be derived from it; for it is to be observed that such rites are the form in which medical knowledge would be preserved amongst a barbarous and ignorant people.
Source Lecture notes of 1818 Lecture XII Miscellaneous criticism Collected works of Coleridge