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