apparently suggested to his successors, Becher and Stahl, the notion of a principle inherent in every combustible substance, which was lost during combustion. The development of this—the phlogistic—theory is almost wholly due to the latter chemist, and indeed it is difficult to trace Becher’s share in it.
George Ernest Stahl was born at Anspach in 1660; he studied and graduated in medicine at Halle, and in 1694 he was appointed second professor of medicine at that University, where he continued to teach for twenty-two years. His most important work was his Fundamenta chymiae dogmaticae et experimentale. His theoretical views are contained in the last part of this work. He there treats of zymotechnia, or fermentation; halotechnia, or the production of salts; and pyrotechnia, or the doctrine of combustion. It is the last of these sections which gives an account of the doctrine of phlogiston.
The fundamental conception of this doctrine is that all combustible bodies are compounds. During combustion one of these constituents, common to all, was dissipated and escaped, while the other, sometimes an acid, sometimes an earthy powder or calx, remained behind. Thus sulphur and phosphorus, when burnt, give acids; and the metals form calces. Non-combustible substances, such as lime, were imagined to be calces, and it was supposed that if phlogiston were restored to them, they too would be converted into metals. This combustible principle was thought to be inherent in all combustible bodies whatsoever; it corresponds in kind with the “sulphur” of more ancient writers, but differs from the latter inasmuch as no very precise ideas were entertained of the identity of the “sulphur” which conferred on the substances containing it as a constituent, or possessing it as a property, their power of combustion. It was also made more definite by Stahl that substances capable of burning or conversion into calces are compounds containing phlogiston in combination with other substances.
Stahl can hardly be credited with more than the invention of the term “phlogiston,” and with bringing the subject in a clear and definite form before his contemporaries. For Stahl wrote in 1720; and we find Mayow, in 1674, entering into an elaborate argument to prove that sulphuric acid is not contained in sulphur, but that it is produced by the union of the sulphur with his fire-air particles. But Stahl amplified the doctrine which Mayow had controverted, in pointing out that if such substances as phosphorus, sulphur, or metals are heated, they burn, and are changed into phosphoric acid, sulphuric acid, or “calces”; and reciprocally, if phosphoric acid, sulphuric acid, or a calx such as that of tin or lead, is heated with matter rich in phlogiston, such as charcoal, pitcoal, sugar, flour, etc., phlogiston is restored to the burnt substance, and the original material, phosphorus, sulphur, tin, or lead, is reproduced. The idea at once captivated the minds of the chemists of that age, who received it with approbation, and devised experiments designed to extend the applications of the theory and to confirm its truth.
Substances were not supposed always to be completely deprived of phlogiston by combustion. Indeed, if the phlogiston were removed wholly, or nearly so, it was by no means easy to restore it. Thus the calx of zinc, or of iron, which was regarded as nearly devoid of phlogiston, is difficult to reduce to the metallic state by ignition with substances rich in phlogiston, such as coal or charcoal. The addition of phlogiston alters the appearance of the substance as regards colour or metallic lustre, and these vary according to the proportion of phlogiston present.
There existed no very definite idea regarding the appearance or properties of phlogiston itself. Becher’s name for it was terra pinguis, and it was represented by Becher and by Stahl as a dry substance of an earthy nature, consisting of very fine particles, which were capable of being set into violent motion; this idea was derived partly from the fact that combustion is usually accompanied by flame, which was supposed to be produced by the motion of the particles of the body, communicated to it by the phlogiston.
It must not be forgotten that at this time it was perfectly well known that metals gain weight on calcination. Jean Rey was quite aware of this, and Boyle relates an experiment to show that tin gains weight when converted into calx; and it will be remembered that Mayow made experiments on the ignition of antimony by the aid of a burning-glass, and rightly conjectured that the substance produced was the same as that formed by treating it with nitric acid, and subsequent ignition. Boyle’s view was that calx of tin was a compound of tin and heat; Mayow’s more correct view was that calx of antimony was a compound of antimony and fire-air. But in spite of these well-proved facts, the adherents of the theory of phlogiston ignored them, and it does not appear to have occurred to Becher or to Stahl that they were inconsistent with their theories.
When this difficulty was stated, which was not until a much later date, a lame explanation of a metaphysical nature, and in itself contradictory, was all that could be offered. It was that phlogiston is endowed with the contrary of gravity or weight, i.e. levity or absolute lightness. This means, of course, that it is repelled by the earth. But if repelled by matter, how comes it that it enters into combination with matter? For it could not remain united if its property were to repel and not to attract. Notwithstanding this, however, the idea satisfied some as to the gain in weight which metals undergo in changing into calces.
It is indeed astonishing that men of such great ability and acumen as Black and Cavendish should have so long lain under the yoke of this absurd theory. It is probable that, in the case of these two great chemists, they stated their results in terms of the theory, partly because they were content to express the facts to which they wished to call attention in this manner, partly because they were not in a position to replace the theory by a more rational one. It is not easy to revolutionise a language, even though its vocabulary be a restricted one. The object of writing is to convey thoughts to others; and it is certainly more convenient to make use of terms understood by others, even if they only imperfectly convey the meaning which it is desired to express, than to attempt a revolution which will probably be unsuccessful, and even if successful, will at all events take time. It is not so difficult to understand Priestley’s attitude, which we shall have to consider later; for Priestley was first of all an experimentalist, and was captivated more by the acquisition of a new fact than by assigning to that fact its proper place in the cosmogony of nature.
The influence of the phlogistic theory on the knowledge of the nature of air was of such a kind as to retard its progress. For how could that knowledge be furthered, when the most active constituent of air was represented by a negation? It may be said that it is easy to be wise after the event—in this case the discovery of oxygen; but here was a theory which was in contradiction to many known facts, and which furnished but a lame explanation of phenomena, and which had been anticipated by another theory, subsequently proved to be correct. Its sole support was the authority of its inventors or adapters, and the deeply-ingrained notions of centuries. We may read from it a lesson that it is wiser to seek out facts which test and prove a theory rather than those which support it, and we may learn for the hundredth time the folly of relying on authority, however ancient and associated with famous names it may be. This was happily expressed by Boyle when he wrote:[3] “For I am wont to judge of opinions as of coins: I consider much less in any one that I am to receive, whose inscription it bears, than what metal ’tis made of. ’Tis indifferent enough to me whether ’twas stamped many years or ages since, or came but yesterday from the mint. Nor do I regard how many or how few hands it has passed through, provided I know by the touchstone whether or no it be genuine, and does or does not deserve to have been current. For if, on due proof, it appears to be good, its having been long, and by many, received for such, will not tempt me to refuse it. But if I find it counterfeit, neither the prince’s image nor superscription, nor the multitude of hands it has passed through, will engage me to receive it. And one disfavouring trial, well made, will much more discredit it with me than all these spurious things I have named can recommend it.”
It has been necessary to enter at some length into the nature of the phlogistic theory, because the discoveries of the time were expressed in its language. The fire-air or vital air of Mayow was termed dephlogisticated air; i.e. air wholly deprived of the power of burning, or air more capable of supporting combustion than ordinary
