From Joseph Priestley
ALS: American Philosophical Society
Leeds. 1 July 1772
I presume that by this time you are arrived in London, and I am willing to take the first opportunity of informing you, that I have niver been so busy, or so successful in making experiments, as since I had the pleasure of seeing you at Leeds.1
I have fully satisfied myself that air rendered in the highest degree noxious by breathing is restored by sprigs of mint growing in it. You will probably remember the flourishing state in which you saw one of my plants. I put a mouse to the air in which it was growing on the saturday after you went, which was seven days after it was put in, and it continued in it five minutes without shewing any sign of uneasiness, and was taken out quite strong and vigorous, when a mouse died after being not two seconds in a part of the same original quantity of air, which had stood in the same exposure without a plant in it. The same mouse also that lived so well in the restored air, was barely recoverable after being not more than one second in the other. I have also had another instance of a mouse living 14 minutes, without being at all hurt, in little more than two ounce measures of another quantity of noxious air in which a plant had grown.2
I have completely ascertained the restoration of air in which, tallow or wax candles, spirit of wine, or brimstone matches have burned out by the same means.3
The nitrous air, which I shewed you, I find to be an admirable test of air that is fit for breathing. It makes this air red and turbid, but no other that I have tried. I took air in which a mouse had putrified, which was in the highest degree noxious and fetid and also a quantity of fixed air. The nitrous air admitted to each of these kinds of air separately made no sensible alteration in them but when they were mixd (which I discovered to make a wholesome air) the nitrous air made the mixture turbid, and diminished the bulk of it, as in common air, tho not in the same degree. A mouse put to this mixture lived five minutes without uneasiness, when, if it had been put to either of them separately, a few minutes before, it would have died in a few seconds.4
Air that has passed thro’ hot charcoal has many, perhaps, all the properties of air that has been diminished by other processes. It extinguishes flame, kills animals, and is not diminished or made turbid by a mixture of nitrous air.5
But the observation that pleases me more than any I ever made, is the diminution of air by the crystallization (I believe) of quicksilver and the nitrous acid. This effect both precedes and follows the generation of nitrous air from the same mixture. This I suspect to be the case with other crystallizations.6
I have observed many other things, which I have not room to mention at present. I am with great respect Dear Sir yours sincerely
Addressed: To / Doctor Franklin / at Mrs Stephenson’s / in Craven Street, in the Strand / London
1. BF did not return to London until July 14. On the 4th, if we read the date correctly, he was at Preston with the Baches, on the 7th and 8th in Birmingham, and at some time during his trip he was in the Lake District. See below, Bache to BF, Oct. 6; BF to Mary Hewson, July 8, and to DF, July 14.
2. Priestley was continuing to investigate the effect that green plants have on the air; see the headnote on the extract of BF’s reply below, under the end of July.
3. An awkward sentence: “by the same means” modifies “restoration.”
4. For Priestley’s earlier observations on “nitrous air,” or nitric oxide, see his letter to BF above, June 13. The use of that “air” in the test described here was of great importance, two years later, in his and Lavoisier’s discovery of what is now called oxygen. See James B. Conant, “The Overthrow of the Phlogiston Theory: The Chemical Revolution of 1775–1789,” Harvard Case Histories in Experimental Science, ed. Conant and Leonard K. Nash (2 vols., Cambridge, Mass., 1957), I, 74–7, 82, 88, 98–103.
5. For his experiments with charcoal that produced this “air,” which he later called phlogisticated air and is the modern nitrogen, see Phil. Trans., LII (1772), 225–7.
6. In this experiment Priestley, without knowing it, was observing oxidation. Combining liquid mercury and “nitrous” (nitric) acid or spirit of nitre produced the “crystals” that he later called red precipitate (an oxide of mercury), generated “nitrous air” (nitric oxide), and reduced the volume of ordinary air. The first reduction, which pleased him so much, occurred when the mixture gave off nitrous oxide, in modern terms, that combined with oxygen in the air to form nitric oxide. The second reduction doubtless occurred in the test, described in the fourth paragraph of the letter, that produced the “red and turbid” air. If Priestley had heated the red precipitate, as he did in 1774, he would have discovered that it gave off an unknown “air,” oxygen. See Conant, op. cit., pp. 94–100.