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From a Paper read to the Philosophical Society of Edinburgh, in 1755, published in the second volume of thePhysical andLiterary Essays , Doctor Black appears to have discovered the affinities between an aëriform substance, which he calledfixed air , and alkalies, quick-lime, and magnesia. His experiments also shewed, that many properties of these bodies depended upon the union and separation of this air. The discovery of these facts established this elastic fluid to be a peculiar species of substance. Mr. Cavendish, Dr. Brownrigg, Dr. Priestley, Sir Torbern Bergman, Mr. Bewley, Mr. Kirwan, and other chemists, afterwards extended, very considerably, the history of fixed air. The question, whether it was a simple or compound body, was discussed; and by many persons it was believed to have been proved, that fixed air was composed of phlogiston and respirable air. But some of the principal facts, upon which this belief was founded, being afterwards demonstrated to be erroneous; and the production of fixed air being, to the apprehension of many chemists, more satisfactorily accounted for by the new principles of chemistry, this doctrine of its composition became no longer tenable. As the science of chemistry advanced, many acids were demonstrably proved to consist of a peculiar basis, and respirable air; and on the ground of analogy it was concluded, that all other acids were composed in a similar manner. Fixed air having been shewn, by Mr. Bewley, and by Berggman, to be an acid, of course its composition was considered, in the new system of chemistry, to be similar to that of all other acids. On examining facts already well ascertained, and by various experiments discovering others, no clear instance could be perceived of theformation of fixed air, but in those cases where charcoal was applied red hot to respirable air. Mr. Lavoisier at last established this interesting fact, by a conclusive experiment, published in a volume of the Memoirs of the Academy of Sciences in 1781, and in hisTraité Elémentaire in 1789, by which he demonstrated that charcoal of wood, except a minute portion of residue, as might reasonably be expected, combined with respirable air, and composed fixed air only. This is the date, therefore, of the discovery by synthesis, of the composition of fixed air; or, as I would rather call it, with Mr. Lavoisier, carbonic acid. The proof by analysis, however, was required, to render the demonstration of the composition of this elastic fluid complete. The honour of the first analytical experiments on carbonic acid is due to Mr. Tennant, F. R. S. who, in a paper read to this Society, in March, 1791, and contained in volume LXXXI. of the Philosophical Transactions, asserted, that charcoal and phosphoric acid were produced by applying phosphorus to red hot marble; from which he inferred, that the carbonic acid of the marble was decompounded. This decomposition, the ingenious author conceives to be effected by the united powers of affinity between phosphorus and the respirable air of the carbonic acid in the calcareous earth, and between the phosphoric acid, thus composed, and the quick-lime of the calcareous earth. That the black matter produced is really charcoal, the author has proved by adequate experiments. The inference, however, does not appear to me to be just, that the charcoal and phosphoric acid are the necessary result of the agency of the affinities, as stated by Mr. Tennant. For the well known fact, that phosphorus cannot be produced from bone-ashes by the application of charcoal and heat, I think, only proves that the powers of affinity between respirable air and phosphorus, together with the affinity between the compound formed by their union (namely, phosphoric acid,) and quick-lime, arenot inferior to the joint affinities between the respirable air, in the phosphoric acid, and charcoal, and between the compound of respirable air and charcoal (namely, carbonic acid) and quick-lime. Hence, from the principle referred to, it could not be concluded, that carbonic acid, combined with quick-lime, would be decompounded by phosphorus attracting its respirable air, and the phosphoric acid, thus formed, attracting the quick-lime. Experience only could determine the result of these affinities, but no proof has been given, from the examination of the mixture, after applying phosphorus to red hot marble; such as finding that carbonic acid was really decompounded, because there was a deficiency of this elastic fluid, and that the charcoal and phosphoric acid corresponded to this deficiency. Accordingly some chemists have conjectured that the small quantity of charcoal afforded in this experiment pre-existed in the phosphorus, which, it is well known, is distilled from charcoal; and others have suspected that it might have arisen from accidental impurities.

XV. Experiments made with the view of decompounding fixed air, or carbonic acid