passive principles. These elements, again, were not definite substances, but merely classes of products obtained by distillation, the active principles being those that passed over and the passive principles those that remained behind in this process—a process which at the time had become the typical process of chemistry, and the chemists of this period are always represented in paintings with a retort or alembic, as were the alchemists of an earlier period with a furnace and crucible. This last enumeration of elements is not so different from that of the alchemists as would at first sight seem, for mercury was regarded as the most active of the spirits, and sulphur as one of the oils. Moreover, the distinction between fixed and volatile oils, which dates from this period, shows the generic character of the elements then accepted.
In his "Œdipus Cymicus," first published about the middle of the seventeenth century, Becher, the author of the theory of phlogiston, comes back to the elements of Aristotle, and in this he is followed by Stahl, who elaborated the same theory a generation later. To give an idea of the confusion of thought on this subject, even at a comparatively late period, I will quote from the "Cours de Chimie, par M. Lémery, nouvelle édition, Paris, 1756," a work which remained one of the chief authorities on chemistry down to the time of Lavoisier. I translate freely from the French:
"The first element of compound bodies which we must accept is a universal spirit, which, being universally diffused, produces different results according as it is held in different matrices or pores of the earth; but as this principle is somewhat metaphysical and can not be perceived by the senses, we must distinguish in addition certain elements which are perceptible. I shall name those commonly accepted.
"As chemists in analyzing different compounds have found five kinds of substances, they have concluded that there are five principles of material things—water, spirit, oil, salt, and earth. Of these five there are three which are active principles—spirit, oil, and salt; and two passive—water and earth. The first are called active, because, being endowed with rapid motion, they determine the active qualities of the products into which they enter; and the second are called passive, because, being at rest, they only serve to diminish the vivacity of the active principles."
Then follows a more precise definition of the several principles enumerated, to which in part I have already referred. After this, Lémery remarks:
"The term principle of chemistry must not be taken in an exact sense, for the substances to which we have given this name are principles only relating to our knowledge, and so far as we have been unable to go further in the division of bodies. But we
