combustible body of whatever kind, which it was one of the chief objects of this theory to establish.

We will endeavour, very briefly, to exemplify the reasoning which Stahl and his followers employed for this purpose.

If phosphorus be burnt in the open air, it gives out light and heat, and is dissipated with a white smoke; but if this process be conducted in a closed vessel, the products of the combustion are collected on its sides, and will rapidly attract moisture from the atmosphere, forming an acid substance called phosphoric acid, and which is considered to be dephlogisticated, or nearly so; but if we proceed to mix it with charcoal powder, and expose it to a strong heat in a glass retort, the phosphorus will be reproduced : and the theory assumes that it is the charcoal which has parted with its phlogiston for this purpose.

It

Again, if instead of charcoal powder we should employ lampblack, or resin, or sugar, or even metallic bodies, and subject them severally, under the same circumstances, to the requisite heat, the same phosphorus would be equally reproduced in every case. was very reasonably concluded, therefore, that it was the same phlogiston which was derived from all those combustible substances, however different in their nature.

A similar succession of phenomena are presented by sulphur. If it be burnt, it forms sulphuric acid; but if the acid thus formed be heated with phosphorus, or charcoal, or coal, or sugar, or even with sulphur itself, it is equally restored to its primitive state.

In all these cases, if one step of the process was granted to be true, the conclusion deducible from the others seemed to be unavoidable. The same effect appeared to be produced, whatever was the source from which the phlogiston was presumed to be derived; and it was thence inferred that the phlogiston, which was thus supplied, was likewise the same principle in all cases.

Metallic bodies also, in the process of calcination or solution, whether by the operation of heat or of acids, presented a series of analogous changes, which were equally calculated to give currency to the same theory.

Thus, if lead be exposed to the requiste heat, it will, in process of time, be reduced to a calx (or oxide) of lead; and if the same lead be placed in concentrated nitric acid, it effervesces violently, and the solution, when evaporated, forms a calx of the same kind: and it was presumed that the same phlogiston is liberated by combustion in one case and by effervescence in the other; but if these calces or oxides are heated again with combustible matters of any kind whatsoever, they are similarly restored to the same metallic

state.

But it is in the operation of double affinities that this theory appeared

appeared to receive one of its most striking illustrations. If a plate of iron be placed in dilute sulphuric acid, it dissolves, and its phlogiston escapes with violent effervescence; but if the same plate be immersed in a solution of copper in the same acid, then it is dissolved with little or no effervescence, transferring its phlogiston tranquilly to the calx of the copper, which is precipitated in its pure metallic form.

In the phlogistic theory the metals and other combustible bodies are considered as compound and their calces as simple, the acid in the preceding case supplying simply the medium through which the affinities act, aiding the separation of the phlogiston from the iron, and its absorption by the calx of the copper: but the modern theory of chemistry would reverse the order of these characters and operations, the acid yielding, under the influence of the predominant affinity, its oxygen to the iron considered as a simple body; and the calx of copper, considered as composed of copper and oxygen, surrendering the second of its constituents to the acid to replace the oxygen which the iron had absorbed.

If we compare these theories with each other, without reference to the relative weights of the metals and their calces, they appear to be equally clear, simple, and satisfactory: they furnish the same results; the medium through which the operations are conducted remains the same in both cases: they are almost equally applicable to the explanation of the infinite variety of facts which chemical agency in its various forms presents to our observation: and it would be difficult to point out, in the history of the sciences, another equally remarkable example of the absolute parallelism of truth and error.

The balance, however, when once applied in these and similar cases, where the process of combustion or solution does not dissipate the materials subjected to its operation, or where their gaseous and other products are carefully collected and weighed, pointed out a consequence which the phlogistic theory was incompetent to explain: the calces or oxides of metal were heavier than the metals from which they were derived; if the calx therefore differs from the derivative metal simply in the loss of its phlogiston, how was this fact to be explained? Does the separation of phlogiston increase the weight of the body from which it escapes? Does this mysterious substance act in opposition to gravity, or does it produce some change in the physical condition of the body with which it was incorporated by which the absolute gravity of its particles is diminished? Newton had shown, both from à priori and à posteriori considerations, that the weight of bodies is proportionate to the mass of matter which they contain; and the Principia' contains few investi

gations

gations which are more refined and beautiful than those by which this very important proposition was established: but chemists were not sufficiently disposed to associate the minuter influences with which they had to deal with the great laws which regulated the movements of the planetary system, and they were little startled with a consequence of their favourite theory, which, if maturely considered, must have been fatal to its truth: even Dr. Black, whose mind had been trained in the strictest habits of inductive philosophy, and who, in his celebrated Essay on the true cause of the causticity of the alkalies, had given so remarkable an example of its correct and rigorous application, was never entirely convinced of the accuracy or relevancy of Newton's reasoning on this subject, and did not consider the argument derived from it against the theory of phlogiston as altogether irrefragable.*

It is less our object, however, to state objections to a theory which we know to be false, than to explain the reason why it was so long considered to be true. Much must undoubtedly be attributed to the influence of the persuasion which had completely occupied the minds of the chemists of all nations that the bases of this theory were satisfactorily established, and that it was fully competent to explain the results of chemical action which had been hitherto observed. Even the great theories of Newton in optics and physical astronomy were not accepted by men of science with a more entire and unhesitating faith: Black, Priestley, Watt, and Kirwan in England, Bergman and Scheele in Sweden, Macquer and Morveau in France, not merely acquiesced in its general truth, but warmly defended it whilst in Germany, the country of its birth, it continued to maintain its empire undisturbed long after it had been elsewhere abandoned as altogether untenable. Cavendish, whose mind had received the most enlarged philosophical training of all the great chemists of his age, whilst he admitted generally its conclusions, cautiously guarded himself against any undue influence which they might exercise upon the character of the deductions which could properly be drawn from his own experiments: it was Lavoisier alone who, from the first opening of his chemical life, struggled with remarkable foresight and undaunted resolution against the principles of the phlogistic theory, when the state of chemical science was not sufficiently advanced to remove very serious objections to his own. It was the great discovery of the

Black's Lectures, by Robison, vol. ii. p. 544. It was in consequence of Dr. Black's doubts on this subject that Dr. Robison repeated Newton's experiments on pendulums with precautions calculated to eliminate any errors which might arise, as had been contended, from uncertainty in the position of the centre of oscillation: it is hardly necessary to say that the results fully confirmed the accuracy of Newton's conclusions. The whole theory has since been made the subject of a most elaborate paper by Bessel in the Berlin Memoirs.

composition

composition of water which alone could clear away the difficulties which opposed the establishment of the true theory.

Again, in the first half of the last century, there were in use no means of accurately examining the products of combustion, or of other chemical operations: no proper apparatus had been invented for collecting, keeping, and examining the gases which escaped in such processes, or the volatile materials which are dissipated if not confined. The balance was rarely appealed to, even when its indications could not be vitiated by the intrusion of foreign products: the influence of the agencies of bodies external to those which were the subjects of chemical action, such as the atmosphere, the various acids and solvents, and more especially water, which it was not easy to exclude, had rarely been attempted to be estimated: the doctrine of the latent and specific heat of bodies, so important from its connexion with the conditions of their existence in a solid, liquid, or aerial form, and of their transition from one of those states to another, was altogether unknown: and various as were the processes of manipulation which had been discovered, remarkable as were the new forms of substances which had been produced, vast as had become the collection of facts which the labours of many ages had accumulated, it would be difficult to quote a single process or experiment, more particularly as regards the operations of chemical affinities, whose circumstances were so clear, definite, and unequivocal as was sufficient to form the basis of a true theory, however rigorously it had been examined, and however strictly the just principles of inductive reasoning had been applied to its explanation.

Neither must it be supposed that the first steps of the great series of researches in pneumatic chemistry, which ended in the discovery of the composition of water and the establishment of the new theory, were very manifestly subversive of the old: one of the most important of these was Dr. Black's discovery of the cause of causticity of the alkalies and of the fixed air which they contain in their mild state. It formed, as is well known, the subject of an Inaugural Dissertation for his degree of M.D., and which was afterwards extended into a separate Essay. Lime and magnesia, in the form which we now call their carbonates, become caustic upon the evolution, by means of heat, of a large quantity of air, which he called fixed air, and which is now called carbonic acid, and they become mild again by its re-union or re-absorption : this gas is easily collected, and it is the same which is evolved in the combustion of charcoal, in the process of fermentation, and in various other natural and chemical operations: it is the same air which is expired by the lungs, and it constitutes the choke-damp

of

of the miners and in every case its presence is detected by its precipitating lime in its mild form, from water which has been previously impregnated by quicklime; in other words, it re-converts quicklime into carbonate of lime, and thus leaves the theory of its composition unquestionable.

Lord Brougham, in the account which he has given of this remarkable discovery, has stated in addition, that Dr. Black determined the fixed air of the alkalies to be heavier than common air: that it possessed acid properties: that it was the first example ever given of the evolution of a permanently elastic fluid, differing from common air not merely in some of its properties, but in its essence: that it was the basis and foundation of all subsequent discoveries in pneumatic chemistry.

A more accurate examination of the facts, however, would show that the first of these properties, and perhaps the most important and distinctive of all those connected with it, was the exclusive discovery of Mr. Cavendish in 1766: that the second was indicated for the first time by Priestley and his fellow-labourers, and only completely established by Lavoisier, who showed fixed air to be carbonic acid, or a combination of oxygen and carbon: it would appear likewise that Hales and others had evolved airs or gases of remarkable properties from various substances both liquid and solid (amongst others the nitrous gas, whose power of diminishing the bulk of atmospheric air Hales had ascertained), leaving it uncertain whether they were compound or simple, or whether their composition was determined by chemical affinity or by the mere admixture of foreign substances with one common and fundamental element, which is the air we breathe. Van Helmont, in the preceding century, had indicated the general identity of the fixed air as evolved from charcoal, or in fermenting liquors, or as it exists in the Grotto del Cane near Naples, under the common name of gas sylvestre, and Dr. Black himself informs us that he was directed to the application of his test of the presence of fixed air in some of those products by the hints given in the works of this visionary but sagacious enthusiast.

The question which thence arises is, what constitutes an essential character of one body, whether gaseous, liquid, or solid, as distinguished from every other? Why should fixed air, which invariably precipitates lime from lime-water, not be a compound of common air with some substance with which it is united or impregnated? Such was the opinion of Kirwan, a chemist of great learning and research, founded upon numerous experiments of Dr. Priestley, who maintained that it was separated * Experiments and Observations concerning the Attractive Powers of the Mineral Acids. Phil. Trans., vol. lxxiii. p. 15.

from