§ =17.= Coming to the alchemists, we find the view that the metals are all composed of two elementary principles--sulphur and mercury--in different proportions and degrees of purity, well-nigh universally accepted in the earlier days of Alchemy. By these terms “sulphur” and “mercury,” however, must not be understood the common bodies ordinarily designated by these names; like the elements of Aristotle, the alchemistic principles were regarded as properties rather than as substances, though it must be confessed that the alchemists were by no means always clear on this point themselves. Indeed, it is not altogether easy to say exactly what the alchemists did mean by these terms, and the question is complicated by the fact that very frequently they make mention of different sorts of “sulphur” and “mercury.” Probably, however, we shall not be far wrong in saying that “sulphur” was generally regarded as the principle of combustion and also of colour, and was said to be present on account of the fact that most metals are changed into earthy substances by the aid of fire; and to the “mercury,” the metallic principle _par excellence_, was attributed such properties as fusibility, malleability and lustre, which were regarded as characteristic of the metals in general. The pseudo-Geber (see § 32) says that “Sulphur is a fatness of the Earth, by temperate Decoction in the Mine of the Earth thickened, until it be hardned and made dry.”[23] He considered an excess of sulphur to be a cause of imperfection in the metals, and he writes that one of the causes of the corruption of the metals by fire “is the Inclusion of a burning Sulphuriety in the profundity of their Substance, diminishing them by Inflamation, and exterminating also into Fume, with extream Consumption, whatsoever Argentvive in them is of good Fixation.”[24] He assumed, further, that the metals contained an incombustible as well as a combustible sulphur, the latter sulphur being apparently regarded as an impurity.[25] A later alchemist says that sulphur is “most easily recognised by the vital spirit in animals, the colour in metals, the odour in plants.”[26] Mercury, on the other hand, according to the pseudo-Geber, is the cause of perfection in the metals, and endows gold with its lustre. Another alchemist, quoting Arnold de Villanova, writes: “Quicksilver is the elementary form of all things fusible; for all things fusible, when melted, are changed into it, and it mingles with them because it is of the same substance with them. Such bodies differ from quicksilver in their composition only so far as itself is or is not free from the foreign matter of impure sulphur.”[27] The obtaining of “philosophical mercury,” the imaginary virtues of which the alchemists never tired of relating, was generally held to be essential for the attainment of the _magnum opus_. It was commonly thought that it could be prepared from ordinary quicksilver by purificatory processes, whereby the impure