Cavendish chose processes for “phlogisticating” air in the course of which no fixed air should be produced. He therefore avoided the use of animal and vegetable materials, and confined himself to combustibles, such as sulphur or phosphorus, to the calcination of metals, the explosion of inflammable air, and the mixture of nitrous air. He adds as a suggestion, “Perhaps it may be supposed that I ought to add to these the electric spark; but I think it much more likely that the phlogistication of the air, and production of fixed air, in this process is owing to the burning of some inflammable matter in the apparatus.” We shall see later what magnificent results arose from this last mode of “phlogisticating” air.
He begins with an account of a repetition of an experiment of Mr. Waltire’s, related by Priestley, in which a mixture of hydrogen and air was exploded in a copper vessel, with the result that they observed a loss of a few grains in weight; it is also stated by Waltire that if the explosion took place in a glass vessel, it became dewy, “which confirmed an opinion he had long entertained, that common air deposits its moisture by phlogistication.” But Cavendish, using a glass vessel of much greater capacity than Waltire’s, could remark no change of weight; and he concluded that 423 measures of hydrogen, or “inflammable air” as he named it, are “nearly sufficient to completely phlogisticate 1000 of common air, and that the bulk of the air remaining after the explosion is then very little more than ⅘ths of the common air employed; so that, as common air cannot be reduced to a much less bulk than that, by any method of phlogistication, we may safely conclude that, when they are mixed in this proportion and exploded, almost all the inflammable air, and about ⅕th part of the common air, lose their elasticity, and are condensed into the dew which lines the glass.
“The better to examine the nature of this ‘dew,’ 500,000 grain measures of inflammable air were burnt with about 2½ times that quantity of common air, and the burnt air made to pass through a glass cylinder 8 feet long and about ¾ of an inch in diameter, in order to deposit the dew”. “By this means upwards of 135 grains of water were condensed in the cylinder, which had no taste or smell, and which left no sensible sediment when evaporated to dryness, neither did it yield any pungent smell during the evaporation; in short, it seemed pure water”. “And by this experiment it appears that this dew is plain water, and consequently that almost all the inflammable and about ⅕th of the common air are turned into pure water.”
But on firing little by little a mixture of “dephlogisticated air” or oxygen, obtained from red precipitate (that is, mercuric oxide prepared by heating the nitrate), with twice its volume of “inflammable air” or hydrogen, the resulting water was acid to the taste, and on evaporation with alkali gave a small quantity—about 2 grains—of nitre. Cavendish suspected that the acid came from the nitrate of mercury in his red precipitate, and, to test this, procured his oxygen from other sources—from red-lead and sulphuric acid, and from the leaves of plants—but still with the same result; nitric acid was formed. Repeating the experiment so as to have present an excess of hydrogen, he found that no acid was produced.
“From the foregoing experiments it appears that when a mixture of inflammable and dephlogisticated air is exploded in such proportion that the burnt air is not much phlogisticated, the condensed liquor contains a little acid, which is always of the nitrous kind, whatever substance the dephlogisticated air is procured from; but if the proportion be such that the burnt air is almost entirely phlogisticated, the condensed liquor is not at all acid, but seems pure water, without any addition whatever; and as, when they are mixed in that proportion, very little air remains after the explosion, almost the whole being condensed, it follows that almost the whole of the inflammable and dephlogisticated air is converted into pure water.” The quantity of uncombined gas was so small that it must be regarded as an impurity. “There can be little doubt that it proceeds only from the impurities mixed with the dephlogisticated and inflammable air, and consequently that if those airs could be obtained perfectly pure, the whole would be condensed.”
The next paragraph is interesting. “During the last summer also [of 1781] a friend of mine gave some account of them [these experiments] to Mr. Lavoisier, as well as of the conclusion drawn from them, that dephlogisticated air is only water deprived of phlogiston; but at that time, so far was Mr. Lavoisier from thinking any such opinion warranted, that, till he was prevailed upon to repeat the experiment himself, he found some difficulty in believing that nearly the whole of the two airs could be converted into water.”
And next comes an important deduction. “Phlogisticated air appears to be nothing else than the nitrous acid united to phlogiston; for when nitre is deflagrated with charcoal, the acid is almost entirely converted into this kind of air.” This is the first statement of the true relation between nitrogen and nitric acid; we should now state the matter by the expression, “Nitrogen is nothing else than nitric acid deprived of oxygen.” And the further deduction is made that “it is well known that nitrous acid is also converted by phlogistication into nitrous air, in which respect there seems a considerable analogy between that and the vitriolic acid; for this acid, when united to a smaller proportion of phlogiston, forms the volatile sulphurous acid and vitriolic acid air, both of which, by exposure to the atmosphere, lose their phlogiston, though not very fast, and are turned back into the vitriolic acid; but when united to a greater proportion of phlogiston, it forms sulphur, which shows no signs of acidity.” “In like manner the nitrous acid, united to a certain quantity of phlogiston, forms nitrous acid and nitrous air, which readily quit their phlogiston to common air; but when united to a different, in all probability a larger quantity, it forms phlogisticated air, which shows no signs of acidity, and is still less disposed to part with its phlogiston than sulphur.”
But the origin of the acid in water made from inflammable and dephlogisticated air was still unexplained. To settle this point Cavendish added to an explosive mixture of oxygen and hydrogen a tenth of its volume of nitrogen, and found that the water was much more strongly acid; and if hydrogen was much in excess, a still greater amount of nitric acid was produced. After relating these experiments he proceeds:—
“From what has been said there seems the utmost reason to think that dephlogisticated air is only water deprived of its phlogiston, and that inflammable air, as was before said, is either phlogisticated water or else pure phlogiston, but in all probability the former.” In a foot-note he gives his reason for the choice, viz. that it requires a red-heat to cause hydrogen and oxygen to combine, while nitrous air combines with oxygen at the ordinary temperature; now, if hydrogen were pure phlogiston, one would expect it to combine more readily than nitrous gas, which has been shown to be a compound of nitric acid with phlogiston. It seems inexplicable that dephlogisticated air should refuse to unite at the ordinary temperature with pure phlogiston, when it is able to extract it from substances with which it has an affinity. Hence it is unlikely that hydrogen is phlogiston itself.
And a few paragraphs farther on Cavendish very nearly discards the phlogistic theory by this statement: “Instead of saying air is phlogisticated or dephlogisticated by any means, it would be more strictly just to say, it is deprived of, or receives, an addition of dephlogisticated air; but as the other expression is convenient, and can scarcely be considered as improper, I shall still frequently make use of it in the remainder of this paper.”