Lecture Xxix.

Of the Doctrine of Phlogiston and the Composition of Water.

It was supposed to be a great discovery of Mr. Stahl, that all inflammable substances, as well as metals, contain a principle, or substance, to which he gave the name of phlogiston, and that the addition or deprivation of this substance makes some of the most remarkable changes in bodies, especially that the union of a metallic calx and this substance makes a metal; and that combustion consists in the separation of phlogiston from the substances that contain it. That it is the same principle, or substance, that enters into all inflammable substances, and metals, is evident, from its being disengaged from any of them, and entering into the composition of any of the others. Thus the phlogiston of charcoal or inflammable air becomes the phlogiston of any of the metals, when the calx is heated in contact with either of them.

On the contrary, Mr. Lavoisier and most of the French chemists, are of opinion, that there is no such principle, or substance, as phlogiston; that metals and other inflammable bodies are simple substances, which have an affinity to pure air; and that combustion consists not in the separation of any thing from the inflammable substance, but in the union of pure air with it.

They moreover say, that water is not, as has been commonly supposed, a simple substance, but that it consists of two elements, viz. pure air, or oxygene, and another, to which they give the name of hydrogene, which, with the principle of heat, called by them calorique, is inflammable air.

The principal fact adduced by them to prove that metals do not lose any thing when they become calces, but only gain something, is, that mercury becomes a calx, called precipitate per se, by imbibing pure air, and that it becomes running mercury again by parting with it.

This is acknowledged: but it is almost the only case of any calx being revived without the help of some known phlogistic substance; and in this particular case it is not absurd to suppose, that the mercury, in becoming precipitate per se, may retain all its phlogiston, as well as imbibe pure air, and therefore be revived by simply parting with that air. In many other cases the same metal, in different states, contains more or less phlogiston, as cast iron, malleable iron, and steel. Also there is a calx of mercury made by the acid of vitriol, which cannot be revived without the help of inflammable air, or some other substance supposed to contain phlogiston: and that the inflammable air is really imbibed in these processes, is evident, from its wholly disappearing, and nothing being left in the vessel in which the process is made beside the metal that is revived by it. If precipitate per se be revived in inflammable air, the air will be imbibed, so that running mercury may contain more or less phlogiston.

The antiphlogistians also say, that the diminution of atmospherical air by the burning of phosphorus is a proof of their theory; the pure air being imbibed by that substance, and nothing emitted from it. But there is the same proof of phosphorus containing phlogiston, that there is of dry flesh containing it; since the produce of the solution of it in nitrous acid, and its effect upon the acid, are the same, viz. the production of phlogisticated air, and the phlogistication of the acid.

Their proof that water is decomposed, is, that in sending steam over hot iron, inflammable air (which they suppose to be one constituent part of it) is procured; while the other part, viz. the oxygene, unites with the iron, and adds to its weight. But it is replied, that the inflammable air may be well supposed to be the phlogiston of the iron, united to part of the water, as its base, while the remainder of the water is imbibed by the calx; and that it is mere water, and not pure air, or oxygene, that is retained in the iron, is evident, from nothing but pure water being recovered when this calx of iron is revived in inflammable air, in which case the inflammable air wholly disappears, taking the place of the water, by which it had been expelled.

In answer to this it is said, that the pure air expelled from the calx uniting with the inflammable air in the vessel, recomposes the water found after this process. But in every other case in which any substance containing pure air is heated in inflammable air, though the inflammable air be in part imbibed, some fixed air is produced, and this fixed air is composed of the pure air in the substance and part of the inflammable air in the vessel. Thus, if minium, which contains pure air, and massicot, which contains none, be heated in inflammable air, in both the cases lead will be revived by the absorption of inflammable air; but in the former case only, and not in the latter, will fixed air be produced. The calx of iron, therefore, having the same effect with massicot, when treated in the same manner, appears to contain no more pure air than massicot does.

Besides this explanation of the facts on which the new theory is founded, which shews it to be unnecessary, the old hypothesis being sufficient for the purpose, some facts are alledged, as inconsistent with the new doctrine.

If the calx of iron made by water, and charcoal made by the greatest degree of heat, be mixed together, a great quantity of inflammable air will be produced; though, according to the new theory, neither of these substances contained any water, which they maintain to be the only origin of it. But this fact is easily explained upon the doctrine of phlogiston; the water in this calx uniting with the phlogiston of the charcoal, and then forming inflammable air; and it is the same kind of inflammable air that is made from charcoal and water.

Also the union of inflammable and pure air, when they are fired together by means of the electric spark, produces not pure water, as, according to the new theory, it ought to do, but nitrous acid.

To this it has been objected, that the acid thus produced came from the decomposition of phlogisticated air, a small portion of which was at first contained in the mixture of the two kinds of air. But when every particle of phlogisticated air is excluded, the strongest acid is procured.

They find, indeed, that by the slow burning of inflammable air in pure air, they get pure water. But then it appears, that whenever this is the case, there is a production of phlogisticated air, which contains the necessary element of nitrous acid; and this is always the case when there is a little surplus of the inflammable air that is fired along with the pure air, as the acid is always procured when there is a redundancy of pure air.

That much water should be procured by the decomposition of these kinds of air, is easily accounted for, by supposing that water, or steam, is the basis of these, as well as of all other kinds of air.

Since air something better than that of the atmosphere is constantly produced from water by converting it into vapour, and also by removing the pressure of the atmosphere, and these processes do not appear to have any limits; it seems probable, that water united to the principle of heat; constitutes atmospherical air; and if so, it must consist of the elements of both dephlogisticated and phlogisticated air; which is a supposition very different from that of the French chemists.