Joseph Priestley (1733-1804)

Objections to the Experiments and Observations relating to the Principle of Acidity, the Composition of Water, and Phlogiston, considered, with further Experiments and Observations on the same Subject

Philosophical Transactions 79, 7- (1789)

Having never failed, says Dr. P., when the experiments were conducted with due attention, to procure some acid whenever I decomposed dephlogisticated and inflammable air in close vessels, I concluded that an acid was the necessary result of the union of those 2 kinds of air, and not water only; which is an hypothesis that has been maintained by Mr. Lavoisier and others, and which has been made the basis of an entirely new system of chemistry, to which a new system of terms and characters has been adapted. The facts that I alleged were not disputed; but to my conclusion it was objected, that the acid I procured might come from the phlogisticated air, which in one of my processes could not be excluded; and that it was reasonable to conclude that this was the case, because Mr. Cavendish had procured the same acid, viz. the nitrous, by decomposing dephlogisticated and phlogisticated air with the electric spark. In other cases it has been said, that the fixed air I procured came from the plumbago in the iron from which my inflammable air had been extracted.

With respect to the former of these objections I would observe, that my process is very different from that of Mr. Cavendish; his decomposition being a very slow one by electricity, and mine a very rapid one by simple ignition, a process by which phlogisticated air, as I found by actual trial, was not at all affected; the dephlogisticated and inflammable airs uniting, and leaving the phlogisticated air (as they probably would any other kind of air with which they might have been mixed) just as it was. I would also observe, that there is no contradiction whatever between Mr. Cavendish's experiment and mine, since phlogisticated air may contain phlogiston, and by means of electricity this principle may be evolved, and unite with the dephlogisticated air, or with the acid principle contained in it, as in the process of simple ignition the same principle is evolved from inflammable air, in order to form the same union; in consequence of which, the water, which was a necessary ingredient in the composition of both the kinds of air, is precipitated. That in other circumstances than those in which I made the experiments, the acid wholly escaped, and nothing but water was found, may be easily accounted for, from the small quantity of the acid principle in proportion to the water, and the extreme volatility of it, owing, I presume, to its high phlogistication when formed in this manner.

In order to ascertain the effect of the presence of phlogisticated air in this process, I now not only repeated the experiment of mixing a given quantity of phlogisticated air with the 2 other kinds of air, and found, as before, that it was not affected by the operation; but I made the experiment with atmospheric air, instead of dephlogisticated. Since the air of the atmosphere contains a greater proportion of phlogisticated air, it might be expected that, if the acid I got before came from the small quantity of phlogisticated air which I could not possibly exclude, I should certainly get more acid when, instead of endeavouring to exclude it, I purposely introduced a greater quantity. But the consequence was the production of much less acid than before, the liquor I procured being sometimes not to be distinguished from pure water, except by the greatest attention possible: for though the decomposition was made in the same copper vessel which I used in the former experiments, there was now no sensible tinge of green colour in it.

When I repeated this experiment in a glass vessel, I perceived, as I imagined, the reason of the small produce of acid in these new circumstances: for the vessel was filled with a vapour which was not soon condensed, and being diffused through the phlogisticated air, (which is not affected by the process) is drawn away along with it, when the exhausting of the tube is repeated; whereas, when there is little or no air in the vessel besides the 2 kinds that unite with each other, and are decomposed, the acid vapour, having nothing to attach itself to and support it (by being entangled with it) much sooner attacks the copper, making the deep green liquor which I have described. Sometimes however I have procured a liquor which was sensibly green by the decomposition of atmospheric and inflammable air, but by no means of so deep a colour, or so sensibly acid, as when the dephlogisticated air is used.

The extreme volatility of the acid thus formed (and which accounts for the escape of some part of it in all of these processes) is apparent from the circumstance, that if the explosions be made in quick succession (the tube being exhausted immediately after each of them, and filled again as soon as possible) no liquor at all will be collected, the whole of the acid vapour, together with the water with which it was combined, being drawn off uncondensed in every process. I once made 20 successive explosions of this kind, in a copper tube, out of which I found that I drew 37 oz. measures of air by the action of the pump, and found not a single drop of liquid, though near an hour was employed in the whole process, and the vessel was never made more than a little warmer than my hand. This was a degree of heat by no mean [sic] sufficient to keep the whole of any quantity of water in a state of vapour; and is a circumstance which of itself sufficiently proves, that the vapour did not consist of water only.

Indeed, I think it impossible for any one to see this vapour in a tall glass vessel, and especially to observe how it falls from one end of it to the other, and the time that is required to its wholly disappearing, without being satisfied that it consists of something else than mere water, the vapour of which would be more equally diffused. If the appearance to the eye should fail to convince any person of this, the sense of smell would do it: for even in a glass vessel it is very offensive, though it might not be pronounced to be acid. I conjecture however that this, and every other species of smell, is produced by some modification of the acid or alkaline principle. Some may be disposed to ascribe this smell to the iron from which the inflammable air was produced; but the smell is the same, or nearly so, when the air is from tin, and would probably be the same if it were from any other substance. Besides using atmospheric air, which contains a greater proportion of phlogisticated air, I have sometimes used dephlogisticated air which was not very pure; and in this case I have always observed, that the liquor I procured had less colour, and was less sensibly acid. These observations might, I should think, satisfy any reasonable person, that the acid liquor which I procured by the explosion of dephlogisticated and inflammable air in close vessels did not come from the phlogisticated air which could not be excluded, whether it was that which remained in the vessel after exhausting it by the air-pump, or that with which the dephlogisticated air was more or less contaminated.

But besides these experiments, in which I procured the green acid liquor by the explosion of dephlogisticated and inflammable air in close vessels, I made another, to which I thought the same objection could not have been made, because no air-pump was used in it, and nothing but the purest dephlogisticated air was employed, being separated in the process from precipitate per se in contact with the purest inflammable air in a glass vessel which had been previously filled with mercury. Accordingly, the only objection made to this experiment was, that the preparation used might be impure, containing something which might yield phlogisticated air. This appeared to me highly improbable, as the precipitate had been made by M. Cadet, and for the purpose of philosophical experiments. Besides, if the heat of a burning lens should dislodge phlogisticated air from any unperceived impurity in this preparation, mere heat will not decompose this air. Let any person try the effect of a lens on such air, or any substance containing it, and produce an acid if he can.

M. Berthollet however, thinking that this might be the case, desired that I would send him a specimen of my precipitate per se. Accordingly, I sent him all that remained of it; and in return he sent me a quantity on the goodness of which I might depend. With this preparation I repeated my former experiment; and, by giving more attention to the process, found it to be far more decisively conclusive in favour of my opinion than I had imagined. In the former experiment I had attended only to the drop of water which was found in the vessel in which the process was made; and finding that it turned the juice of turnsole red, I concluded that it contained nitrous acid: but I now examined the air that remained in the vessel, and found that a considerable proportion of it was fixed air, so that I am now satisfied this was the acid with which it was impregnated, and not the nitrous. Still however some acid is the constant result of the union of the two kinds of air, and not water only. A quantity of the same precipitate per se yielded no fixed air by heat. Comparing this experiment with that in which iron is ignited in dephlogisticated air, this general conclusion may be drawn, viz. that when either inflammable or dephlogisticated air is extracted from any substance in contact with the other kind of air, so that one of them is made to unite with the other in what may be called its nascent state, the result will be fixed air; but that if both of them be completely formed before their union, the result will be nitrous acid.

It has been said, that the fixed air produced in both these experiments may come from the plumbago in the iron from which the inflammable air is obtained. But since we ascertain the quantity of plumbago contained in iron by what remains after its solution in acids, it is in the highest degree improbable, that whatever plumbago there may be in iron, any part of it should enter into the inflammable air procured from it. Besides, according to the antiphlogistic hypothesis, all inflammable air comes from water only. As it cannot be said, that any real fixed air is found in inflammable air from iron (since it is not discoverable by lime-water) it must be supposed, that the elements, or component parts of fixed air are in it; but one of these elements is pure air, and the mixture of nitrous air shows that it contains no such thing, though, according to M. Lavoisier, fixed air contains 72 parts in 100 of pure air.

However, being apprized of this objection to inflammable air form iron, I made use of inflammable air from tin, and I had the same result as with that from iron. I also calculated the weight of the fixed air which I got in the process, and comparing it with the plumbago which the iron necessary to make the inflammable could have contained, I found that in all the cases it far exceeded the weight of the plumbago; so that it was absolutely impossible, that the fixed air which I found should have had this origin. For the greater satisfaction, Dr. P. recites the particulars of a few experiments of this kind. But, for these, we must refer to a separate pamphlet which Dr. P. published on this subject.

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