Stahl devised a new name for this combustible constituent of substances, in order better to direct attention to his new idea; he called it " phlogiston," a word which may be translated " burnableness/' for it is derived from a Greek word signifying flame. Thus lead, which when heated in air changes to an earthy dross, or, as it was then termed, "calx," may be restored to the state of metal by heating it with charcoal powder, or with flour, or with any substance rich in " phlogiston ; " that is, with any substance which is itself capable of burning. He supposed that the lead was rich in " phlogiston ;³⁹ that when it changed to lead-dross, the " phlogiston " escaped ; but that on heating with charcoal, the latter parted with its phlogiston to the lead-dross, changing it back again into lead. It is evident that this idea accounts for some of the facts observed; and it gained ground rapidly. But it had been shown by the French physician Jean Rey, by the English philosopher Robert Boyle, and others, that metals, when they changed into earthy powders on heating, gained weight; and it is at least curious that the lead, on losing one of its constituents, namely "phlogiston," should gain weight: one would have expected that weight would be lost, not gained. The way out of this difficulty was ingenious. We know, it was said, that weight is due to the attraction of the earth ; now, it is not impossible that the earth may repel phlogiston, instead of attracting it; and in that case a body would grow heavier, instead of lighter, if it parts with phlogiston. Another objection to the theory was that a candle, for example, which is rich in phlogiston, goes out when made to burn under a glass shade; that is, when air is excluded. True, said the phlogistonists; that is because the phlogiston cannot escape. And because this theory gave a plausible explanation of the common phenomenon of combustion, it was widely accepted, and survived until the end of the eighteenth century.

The idea had been steadily gaining ground that knowledge was to be acquired only by trial and failure. Francis Bacon, Lord Verulam, at the end of the sixteenth century wrote: "The true kind of experience is not the mere groping of a man in the dark, who feels at random to find his way, instead of waiting for the dawn or striking a light. ... It begins with an ordered—not chaotic— arrangement of facts, deduces axioms from these, and from the axioms again designs new experiments." Many experiments were made on the action of heat on various things, either alone or mixed with others. Boyle, who introduced the word "analysis" to denote the art of discovering one substance in presence of another, and who contended for the use of the word "element" in the meaning of a constituent of, and not as a. property of matter, made many such experiments, and systematically put Bacon's advice into practice. And so knowledge of facts gradually accumulated, and speculations acquired some substantial basis. To Priestley, a nonconformist clergyman of Birmingham, we owe the discovery of numerous gases, and Scheele, his contemporary, a Swedish apothecary, also enriched chemistry in this respect. The discovery of oxygen, in 1774, was made simultaneously by both of these illustrious men. It had been generally supposed that all gases, or, as they were then termed, "airs," were merely modifications of atmospheric air; and it was not uncommonly held that air, in consequence of its want of substantiality, was but one remove from nothing. Joseph Black, Professor of Chemistry in Edinburgh in the middle of the eighteenth century, was the first to prepare carbonic acid gas, or, as he termed it, "fixed air," in a pure, state ; and by determining the loss of weight on heating its compound with magnesia, to show that it was due to the escape of the -gas; for he succeeded also in absorbing the gas, and reconstituting the carbonate of magnesia, which then possessed practically the same weight as it originally had. In spite of this discovery, made in 1756, the doctrine was still generally held that burning substances lost their constituent principle, "phlogiston ; " and we owe to the French chemist Lavoisier the true explanation of the phenomenon of combustion. Lavoisier had been informed by Priestley in the autumn of 1774 of his discovery of what, according to the views then current, he termed " dephlogisticated air; " he proceeded to repeat an experiment which had previously been made by Boyle, in heating metallic tin to redness in a sealed glass vessel; there was neither gain nor loss of weight, although the tin had been partly converted into " calx; " but on admitting air, he observed a gain in weight, nearly equal to that which the tin had gained on being calcined. The conclusion was obvious, that the gain in weight was due to the absorption of a portion of the air by the hot tin; and he subsequently showed that the gain was to be ascribed to the absorption of Priestley's " de-phlogisticated air," of which Priestley had shown common air to contain about one-fifth. And in 1777 Lavoisier published the statements:—

(1) Substances burn only in pure air.

(2) This air is consumed in the combustion, and the increase in weight of the substance burned is equivalent to the decrease in weight of the air.

(3) The combustible body is, as a rule, converted into an acid by its combination with the pure air, but the metals, on the other hand, are converted into " calces."