ties of substances which form the foundation of the science, and constitute Liebig's first stage in its progress.
Although the second stage—the formation of general ideas or theories—is a sequence of the first, and implies accumulated observations to be explained, yet it was begun early. The doctrine of the four primitive elements, fire, air, earth, and water, was the first chemical theory, and sufficed for many centuries. To these four elements of Aristotle, which were regarded as the four fundamental causes of the physical properties of matter, were added three new elements—mercury, sulphur, and salt—which also stood for certain properties and causes of change, rather than concrete bodies. Mercury represented volatility, and was supposed to give this property to matter; sulphur was connected with changeableness by fire, or combustibility, and salt represented fixity, like the salts found in ashes. On this view, alcohol, or aqua vitæ, was regarded as "sulphurous vegetable mercury," which only meant that it was inflammable and volatile. Hence Basil Valentine says: "When a rectified aqua vitæ is kindled, its mercury and sulphur separate; the sulphur burns quite vividly, for it is pure fire, and the delicate mercury flies into the air and returns to its original chaos."
Such rude ideas answered to begin the work of chemical theorizing, but the increase of facts at length showed that they were contradictory and absurd. About a hundred years before the time of Priestley, Beccher, a German chemist, in undertaking to correct the doctrine of salt, sulphur and mercury, struck a new conception which soon grew into a comprehensive and important chemical theory. In working with sulphur, he sagaciously detected the analogy between the formation of sulphuric acid from sulphur and the reduction of metals to an earthy form (calx). The metal was supposed to consist of an earth, and something which, in the process of combustion, was separated from it; in like manner sulphur was supposed to consist of an acid and something that was separated from it, by burning, and to this something Stahl afterward gave the name of phlogiston—Greek for combustible. So intimately and extensively were fire and combustion involved with chemical changes, that a theory of combustion was regarded as the same thing as a theory of chemistry. It was assumed that all combustible bodies are compounds. One of the constituents was supposed to be dissipated during the process, while the other remained behind. The part dissipated, phlogiston, was held to be the same in all combustible bodies whatever, and hence the differences among them depended upon the residues. On this view, the property of combustibility is always owing to the presence of phlogiston, and fire, or inflammation, to its escape. Phlogiston was communicable from body to body. When phosphorus is burned it loses its phlogiston, and an acid remains. But if now the acid is heated in a retort with charcoal-powder, sugar, or resin, these combustibles are deprived of their phlogiston, which, passing over to the acid, reproduces phosphorus. Bodies saturated with phlogiston were said to be phlogisticated, and, when deprived of it, were dephlogisticated, processes which might be as partial or complete as the variations of combustive phenomena. These ideas were founded upon experiments so decisive that, when the existence of the principle itself was once admitted, the explanation was entirely satisfactory. "There are ideas," says Liebig, "so great and vast that, even when entirely perforated, as it were, in all directions, they leave enough of matter to occupy the powers of thought of mankind for a century. Such a vast idea was that of phlogiston. The question as to its material existence was void of
