When the alchemists described an element or principle as male or female, they meant what they said, as I have already intimated, to the extent, at least, of firmly believing that seed was produced by the two metallic sexes. By their union metals were thought to be produced in the womb of the earth; and mines were shut in order that by the birth and growth of new metal the impoverished veins might be replenished. In this way, too, was the _magnum opus_, the generation of the Philosopher's Stone--in species gold, but purer than the purest--to be accomplished. To conjoin that which Nature supplied, to foster the growth and development of that which was thereby produced; such was the task of the alchemist. "For there are Vegetables," says BERNARD of TREVISAN in his _Answer to Thomas of Bononia_, "but Sensitives more especially, which for the most part beget their like, by the Seeds of the Male and Female for the most part concurring and conmixt by copulation; which work of Nature the Philosophick Art imitates in the generation of gold."(1) (1) _Op. cit_., p. 216. Mercury, as I have said, was commonly regarded as the seed of the metals, or as especially the female seed, there being two seeds, one the male, according to BERNARD, more ripe, perfect and active, the other the female. "more immature and in a sort passive(2) "... our Philosophick Art," he says in another place, following a description of the generation of man, "... is like this procreation of Man; for as in _Mercury_ (of which Gold is by Nature generated in Mineral Vessels) a natural conjunction (2) _Ibid_., p. 217; _cf_. p. 236 is made of both the Seeds, Male and Female, so by our artifice, an artificial and like conjunction is made of Agents and Patients."(1) "All teaching," says KELLY, "that changes Mercury is false and vain, for this is the original sperm of metals, and its moisture must not be dried up, for otherwise it will not dissolve,"(2) and quotes ARNOLD (_ob. c_. 1310) to a similar effect.(3) One wonders how far the fact that human and animal seed is fluid influenced the alchemists in their choice of mercury, the only metal liquid at ordinary temperatures, as the seed of the metals. There are, indeed, other good reasons for this choice, but that this idea played some part in it, and, at least, was present at the back of the alchemists' minds, I have little doubt.