James Watt (1736-1819)

Thoughts on the Constituent Parts of Water and of Dephlogisticated Air; with an Account of some Experiments on that Subject

excerpt from Phil. Trans. 74, 329 (1784).

1. It has been known for some time, that inflammable air contained much phlogiston; and Dr. Priestley has found, by some experiments made lately, that it "is either wholly pure phlogiston, or at least that it contains no apparent mixture of any other matter." In my opinion however, it contains a small quantity of water, and much elementary heat.[1] He found, that by exposing the calces of metals to the solar rays, concentrated by a lens, in a vessel containing inflammable air only, the calces of the softer metals were reduced to their metallic state; and that the inflammable air was absorbed in proportion as they became phlogisticated; and, by continually supplying the vessel with inflammable air, as it was absorbed, he found, that out of 101 oz. measures, which he had put into the vessel, 99 oz. measures were absorbed by the calces, and only 2 oz. measures remained, which, on examination, he found to be nearly of the same quality the whole quantity had been of before the experiment, and to be still capable of deflagrating in conjunction with atmospheric or with dephlogisticated air. Therefore, as so great a quantity of inflammable air had been absorbed by the metallic calces; the effect of reducing them to their metallic state had been produced; and the small remaining portion was still unchanged, at least had suffered no change which might not be attributed to its original want of purity; it was reasonable to conclude, that inflammable air must be the pure phlogiston, or this matter which reduced the calces to metals.

2. The same ingenious philosopher mixed together certain proportions of pure dry dephlogisticated air and of pure dry inflammable air in a strong glass vessel, closely shut, and then set them on fire by means of the electric spark, in the same manner as is done in the inflammable air pistol. "The first effect was the appearance of red heat or inflammation in the airs, which was soon followed by the glass vessel becoming hot. The heat gradually pervaded the glass, and was dissipated in the circumambient air, and as the glass grew cool, a mist or visible vapour appeared in it, which was condensed on the glass in the form of moisture or dew.[2] When the glass was cooled to the temperature of the atmosphere, if the vessel was opened with its mouth immersed in water or mercury, so much of these liquids entered, as was sufficient to fill the glass within about 1/200 part of its whole contents; and this small residuum may safely be concluded to have been occasioned by some impurity in one or both kinds of air. The moisture adhering to the glass, after these deflagrations, being wiped off, or sucked up, by a small piece of sponge paper, first carefully weighed, was found to be exactly, or very nearly, equal in weight to the airs employed. In some experiments, but not in all, a small quantity of a sooty-like matter was found adhering to the inside of the glass," the origin of which is not yet investigated; but Dr. Priestley thinks, that it arises from some minute grains of the mercury that was used to fill the glass with the air, which being super-phlogisticated by the inflammable air, assumed that appearance: but, from whatever cause it proceeded, the whole quantity of sooty-like matter was too small to be an object of consideration, particularly as it did not occur in all the experiments.

I am obliged to Mr. De Luc for the account of the experiments which have been lately made at Paris on this subject, with large quantities of these 2 kinds of air, by which the essential point seems to be clearly proved, that the deflagration or union of dephlogisticated and inflammable air, by means of ignition, produces a quantity of water equal in weight to the airs; and that the water thus produced appeared, by every test, to be pure water. As I am not furnished with any particulars of the manner of making the experiment, I can make no observations on it, only that from the character of the gentlemen who made it, there is no reason to doubt of its being made with all necessary precautions and accuracy, which was further secured by the large quantities of the 2 airs consumed.

3. "Let us now consider what obviously happens in the case of the deflagration of the inflammable and dephlogisticated air. These 2 kinds of air unite with violence, they become red-hot, and on cooling totally disappear. When the vessel is cooled, a quantity of water is found in it equal to the weight of the air employed. This water is then the only remaining product of the process, and water, light, and heat, are all the products," unless there be some other matter set free which escapes our senses. "Are we not then authorised to conclude, that water is composed of dephlogisticated air and phlogiston, deprived of part of their latent or elementary heat; that dephlogisticated or pure air is composed of water deprived of its phlogiston, and united to elementary heat and light; and that the latter are contained in it in a latent state, so as not to be sensible to the thermometer or to the eye; and if light be only a modification of heat, or a circumstance attending it, or a component part of the inflammable air, then pure or dephlogisticated air is composed of water deprived of its phlogiston and united to elementary heat?"

4. "It appears, that dephlogisticated water," or, which may be a better name for the basis of water and air, the element Mr. De Luc calls humor, "has a more powerful attraction for phlogiston than it has for latent heat, but that it cannot unite with it, at least not to the point of saturation, or to the total expulsion of the heat, unless it be first made red-hot," or nearly so. "The electric spark heats a portion of it red- hot, the attraction between the humor and the phlogiston takes place, and the heat which is let loose from this first portion heats a second, which operates in a like manner on the adjoining particles, and so continually till the whole is heated red-hot and decomposed." Why this attraction does not take place to the same degree in the common temperature of the atmosphere, is a question I am not yet able to solve; but it appears, that, in some circumstances, "dephlogisticated air can unite, in certain degrees, with phlogiston without being changed into water." Thus Dr. Priestley has found, that by taking clean filings of iron, which alone produce only inflammable air of the purest kind, and mercurius calcinatus per se, which gives only the purest dephlogisticated air, and exposing them to heat, in the same vessel, he obtained neither dephlogisticated non inflammable air, "but in their place fixed air." Yet it is well known, that a mixture being as capable of deflagration at the end of that time as it was when first shut up. These facts the Doctor accounts for, by supposing that the 2 kinds of air, when formed at the same time in the same vessel, can unite in their nascent state; but that, when fully formed, they are incapable of acting on each other, unless they are first set in motion by external heat. Phlogisticated air seems also to be another composition of phlogiston and dephlogisticated air; but in what proportions they are united, or by what means, is still unknown. It appears to be very probable, that fixed air contains a greater quantity of phlogiston than phlogisticated air does, because it has a greater specific gravity, and because it has more affinity with water.

5. For many years I have entertained an opinion, that air was a modification of water, which was originally founded on the facts that in most cases, wherein air was actually made, which should be distinguished from those wherein it is only extricated from substances containing it in their pores, or otherwise united to them in the state of air, the substances were such as were known to contain water as one of their constituent parts, yet no water was obtained in the processes, except what was known to be only loosely connected with them, such as the water of the crystallization of salts. This opinion arose from a discovery, that the latent heat contained in steam diminished in proportion as the sensible heat of the water from which it was produced increased; or, in other words, that the latent heat of steam was less when it was produced under a greater pressure, or in a more dense state, and greater when it was produced under a less pressure, or in a less dense state; which led me to conclude, that when a very great degree of heat was necessary for the production of the steam, the latent heat would be wholly changed into sensible heat; and that, in such cases the steam itself might suffer some remarkable change. I now abandon this opinion in so far as relates to the change of water into air, as I think that may be accounted for on better principles.

...[Numbers 6 through 14 and most of 15 are omitted.--CJG]

Mr. Scheele has formed an hypothesis on this subject, in which he supposes heat to be composed of dephlogisticated air united to phlogiston, and that this combination is sufficiently subtile to pass through glass vessels. He affirms, that the nitrous and other acids, when in an ignited state, attract the phlogiston from the heat, and set the dephlogisticated heat at liberty; but he does not seem to have been more successful than myself in explaining what becomes of the acid of nitre and phlogiston in the case of the decomposition of nitre by heat. And since we know, from the late experiments, that water is a composition of air, or more properly humour and phlogiston, his whole theory must fall to the ground, unless the fact be otherwise accounted for, which it does not seem easy to do.

16. To return to the experiment of the deflagration of dephlogisticated and inflammable air, it appears from the two airs becoming red-hot on their union, that the quantity of heat contained in one or both of them, is much greater than that contained in steam; because, for the first moments after the explosion, the water deposited by the air remains in the form of steam, and consequently retains the latent heat due to that modification of water. This matter may be easily examined by firing the mixture of dephlogisticated and inflammable air in a vessel immersed in another vessel containing a given quantity of water of a known heat, and after the vessel in which the deflagration is performed is come to the same temperature with the water in which it is immersed, by examining how much heat that water has gained, which being divided by the quantity of water produced by the decomposition of the airs, will give the whole quantity of elementary or latent heat which that water had contained, both as air and as steam; and if from that quantity we deduct the latent heat of the steam, the remainder will be the latent or elementary heat contained more in air than in steam. This experiment may be made more completely by means of the excellent apparatus which Messrs. Lavoisier and De la Place have contrived for similar purposes.

[1] Previous to Dr. Priestley's making these experiments, M. Kirwan had proved, by very ingenious deductions from other facts, that inflammable air was, in all probability, the real phlogiston, in an aerial form. These arguments were perfectly convincing to me; but it seems more proper to rest that part of the present hypothesis on the direct experiment.

[2] I believe that Mr. Cavendish was the first who discovered that the combustion of dephlogisticated and inflammable air produced moisture on the sides of the glass in which they were fired.

Back to the list of selected historical papers.
Back to the top of Classic Chemistry.