And now we come to the consideration of Lavoisier’s new theory, and its rejection in favour of the old one of phlogiston. It is curious to follow the reasoning which made such an exceptionally acute thinker as Cavendish deliberately reject the true explanation. Cavendish first states his results in Lavoisier’s terms:—
“According to this hypothesis, we must suppose that water consists of inflammable air united to dephlogisticated air; that nitrous air, vitriolic acid air (sulphur dioxide), and the phosphoric acid are also combinations of phlogisticated air, sulphur, and phosphorus with dephlogisticated air; and that the two former, by a further addition of the same substance, are reduced to the common nitrous and vitriolic acids; that the metallic calces consist of the metals themselves united to the same substance, commonly, however, with a mixture of fixed air; that on exposing the calces of the perfect metals to a sufficient heat, all the dephlogisticated air is driven off, and the calces are restored to their metallic form; but as the calces of the imperfect metals are vitrified by heat, instead of recovering the metallic form, it should seem as if all the dephlogisticated air could not be recovered from them by heat alone. In like manner, according to this hypothesis, the rationale of the production of dephlogisticated air from red precipitate is, that during the solution of the quicksilver in the acid and the subsequent calcination, the acid is decompounded, and quits part of its dephlogisticated air to the quicksilver, whence it comes over in the form of nitrous air, and leaves the quicksilver behind united to dephlogisticated air, which, by a further increase of heat, is driven off, while the quicksilver resumes its metallic form. In procuring dephlogisticated air from nitre, the acid is also decompounded; but with this difference, that it suffers some of its dephlogisticated air to escape, while it remains united to the alkali itself in the form of phlogisticated nitrous acid. As to the production of dephlogisticated air from plants, it may be said that vegetable substances consist chiefly of three different bases, one of which [hydrogen], when united to dephlogisticated air, forms water; another [carbon] fixed air; and the third phlogisticated air [nitrogen]; and that, by means of vegetation, each of these substances are decomposed, and yield their dephlogisticated air; and that, in burning, they again acquire dephlogisticated air, and are restored to their pristine form.
“It seems, therefore, from what has been said, as if the phenomena of nature might be explained very well on this principle, without the help of phlogiston; and indeed, as adding dephlogisticated air to a body comes to the same thing as depriving it of its phlogiston and adding water to it, and as there are perhaps no bodies destitute of water, and as I know no way by which phlogiston may be transferred from one body to another, without leaving it uncertain whether water is not at the same time transferred, it will be very difficult to determine by experiment which of these opinions is the truest; but as the commonly-received principle of phlogiston explains all phenomena, at least as well as Mr. Lavoisier’s, I have adhered to that.”
“Another thing which Mr. Lavoisier endeavours to prove is that dephlogisticated air is the acidifying principle. From what has been explained, it appears that this is no more than saying that acids lose their acidity by uniting to phlogiston, which, with regard to the nitrous, vitriolic, phosphoric, and arsenical acids, is certainly true”. “But as to the marine acid and acid of tartar, it does not appear that they are capable of losing their acidity by any union with phlogiston.”
Here Cavendish does not consider the question of gain of weight on loss of phlogiston, or if he does, he must ascribe it to simultaneous entry of water. And experimental research at that time was not far enough advanced to enable him to decide finally as to the truth of this hypothesis.
In his next memoir, read before the Royal Society on June 2nd, 1785, Cavendish relates experiments on the passage of electric sparks through air, the experiment having first been tried by Priestley. Priestley says:[24]— “Lastly, the same effect [i.e. the diminution of the volume of common air], I find, is produced by the electric spark, though I had no expectation of this event when I made the experiment.” And again:—“At the time of my former publication, I had found that taking the electric spark in given quantities of several kinds of air had a very remarkable effect on them, that it diminished common air and made it noxious, making it deposit its fixed air exactly like any phlogistic process; from whence I concluded that the electric matter either is or contains phlogiston.”
Cavendish had mentioned this process casually as one of the methods of phlogisticating air; in beginning his second paper he says:—“I now find that, though I was right in supposing the phlogistication of the air does not proceed from phlogiston communicated to it by the electric spark, and that no part of the air is converted into fixed air; yet that the real cause of the diminution is very different from what I suspected, and depends upon the conversion of phlogisticated air into nitrous acid.” The apparatus he used was very simple. It consisted of a glass siphon filled with mercury, each leg dipping into a glass likewise containing mercury; the air was admitted by a gas-pipette into the bend of the siphon, and on connecting the mercury in one of the glasses with a ball placed near the prime conductor of an electric machine, and the other with the earth, sparks could be made to pass from the mercury in one limb to that in the other.
The product obtained by passing sparks through air in this manner turned litmus red, and gave rise to no cloud in lime-water, while the air was reduced to two-thirds of its original volume; nor did the lime-water give a precipitate on introducing some fixed air, this showing that it had been saturated by an acid. It was found, too, that “soap-lees,” or solution of caustic potash, if present, diminished the volume more rapidly than did lime-water; and repeated trials proved that “when five parts of pure dephlogisticated air were mixed with three parts of common air, almost the whole of the air was made to disappear.” The nitrate of potassium thus produced caused paper soaked in it and dried to deflagrate; and it contained no sulphuric acid. “There is no reason to think that any other acid entered into it except the nitrous.” But it gave a precipitate with silver nitrate; and Cavendish, suspecting that this was silver nitrite, prepared some potassium nitrite by heating the nitrate; on comparing the white precipitate which this solution gave with silver nitrate with that obtained from his “soap-lees,” he found them identical. There was therefore no “muriatic acid” present, which would have yielded chloride of silver, of appearance somewhat similar to the nitrite.
As it had previously been shown to be probable that phlogisticated air is nitrous air united with phlogiston, and that nitrous air is nitric acid united with phlogiston, “we may safely conclude that in the present experiments the phlogisticated air was enabled, by means of the electric spark, to unite to, or form a chemical combination with, the dephlogisticated air, and was thereby reduced to nitrous acid, which united to the soap-lees and formed a solution of nitre; for in these experiments the two airs actually disappeared, and nitrous acid was actually formed in their room”. “A further confirmation of the above-mentioned opinion is that, as far as I can perceive, no diminution of air is produced when the electric spark is passed either through pure dephlogisticated air or through perfectly phlogisticated air, which indicates a necessity of a combination of these two airs to produce the acid. Moreover, it was found in the last experiment that the quantity of nitre procured was the same that the soap-lees would have produced if saturated with nitrous acid; which shows that the production of the nitre was not owing to any decomposition of the soap-lees.”
Nothing more clearly shows the care with which Cavendish reasoned than these last quotations. No loophole is left unstopped; every precaution is taken to make the proof as faultless as it is possible for a proof to be.