When a confined volume of common air is placed in contact with a mixture of iron filings and sulphur made into a paste with water, a certain portion of the air is imbibed by the paste. This fact was first observed by Hales. Priestley repeated the observation and found that about a fifth or rather more of the volume of the air was thus absorbed. He noted that the residual “air” was rather lighter than common air, it had no action on lime-water and was exceedingly noxious to animals, by which is meant that it could not be breathed by them. Priestley had thus prepared nitrogen, but he failed to recognise the individuality of this gas.
In his Statical Essays Hales makes mention of an experiment in which common air and air generated from pyrites by spirit of nitre made a turbid red mixture, and in which part of the common air was absorbed. This phenomenon “particularly struck” Priestley, who, acting upon Cavendish’s hint that the red appearance was probably dependent “upon the spirit of nitre only” and that the metals might answer as well as pyrites, proceeded to investigate the action of nitric acid upon a number of the metals, and as the result of his inquiries he succeeded in isolating the gas we now know as nitric oxide, but which he termed nitrous air.
“Though,” he says, “I cannot say that I altogether like the term, neither myself nor any of my friends, to whom I have applied for the purpose, have been able to hit upon a better.”
This paper exhibits Priestley at his best. In it he describes all the main properties of nitric oxide.
“One of the most conspicuous properties of this kind of air,” he says, “is the great diminution of any quantity of common air with which it is mixed, attended with a turbid red, or deep orange colour, and a considerable heat.... The diminution of a mixture of this and common air is not an equal diminution of both the kinds, which is all that Dr Hales could observe, but of about one fifth of the common air, and as much of the nitrous air as is necessary to produce that effect; which, as I have found by many trials, is about one half as much as the original quantity of common air.
“I hardly know any experiment that is more adapted to amaze and surprise than this is, which exhibits a quantity of air which, as it were, devours a quantity of another kind of air half as large as itself, and yet is so far from gaining any addition to its bulk that it is considerably diminished by it....
“It is exceedingly remarkable that this effervescence and diminution, occasioned by the mixture of nitrous air, is peculiar to common air, or air fit for respiration, and, as far as I can judge from a great number of observations, is at least very nearly, if not exactly, in proportion to its fitness for this purpose; so that by this means the goodness of air may be distinguished much more accurately than it can be done by putting mice or any other animals to breathe in it.
“This was a most agreeable discovery to me, as I hope it may be a useful one to the public; especially as from this time I had no occasion for so large a stock of mice as I had been used to keep for the purpose of these experiments.”
Priestley here suggests the basis of a method of Eudiometry, or method of measuring the goodness of air, which in his hands, but more especially in those of Cavendish, led to most important results. The quantitative analysis of the air may be said to have taken its rise from the publication of Priestley’s paper.
In the course of subsequent work on nitrous air Priestley had occasion to study its action on iron, whereby he says:—
“A most remarkable and most unexpected change was made in the nitrous air,” the iron “makes it not only to admit a candle to burn in it, but enables it to burn with an enlarged flame.... Sometimes I have perceived the flame of the candle, in these circumstances, to be twice as large as it is naturally, and sometimes not less than five or six times larger; and yet without anything like an explosion, as in the firing of the weakest inflammable air.”
Priestley in this manner obtained nitrous oxide, the properties of which he subsequently studied in some detail.
In the paper which follows, viz., “On Air infected with the Fumes of Burning Charcoal,” he incidentally gains further insight into the nature of atmospheric air. By what he called throwing the focus of a burning mirror on charcoal suspended in air contained in a glass tube standing over water or mercury—a favourite method of his when he had occasion to heat a substance in a gas—he could observe the phenomena with great precision. He noticed the formation of the fixed air and determined the degree of diminution when the burning took place over water or over lime-water.