Benjamin Franklin Papers

To Benjamin Franklin from Joseph Priestley, 26 September 1773

From Joseph Priestley

ALS: American Philosophical Society

Calne3 26 Septr 1773.

Dear Sir

With this I return you Mr. Winthrop’s letter, according to your desire, thanking you for your endeavours to serve me in America, though I find, as I was apprehensive, that the scheme would not answer.4 Please to return my thanks to the professor for his candid and judicious remarks on my History of Opticks, which will be much improved by them, if it should come to a second edition.

Dr. Price will have informed you, that I have resumed my experiments on air, and with a good prospect of success. Since that I have been much more successful; but in a letter I can only confine myself to the heads of things.

The most important of the observations I have made lately is of an alkaline, corresponding to my acid air, of which an account is given in what I have already printed. This I get by treating a volatile alkali in the same manner in which I before treated the spirit of salt. As soon as the liquid begins to boil the vapour arises, and being received in a vessel filled with quick-silver, continues in the form of air, not condensed by cold.5

I imagined that a mixture of this alkaline with acid air would make a neutral, and perhaps a common air; but, instead of that, they make a beautiful white salt of a very curious nature. It immediately deliquesces and even wholly disappears upon being exposed to the open air, and if it be in a dry and deep vessel, where moisture cannot easily come at it, it wholly evaporates in dense white clouds, occasioning a very strong smell.6

This Alkaline vapour, like the acid, is quickly imbibed by water, which thereby becomes Spirit of Sal Volatile.

Nitrous air makes this alkaline vapour turbid,7 and perhaps generates a different salt; but I have not yet made a tenth part of the experiments that I propose to do with this new kind of air.

I have just found that spirit of wine yields air also, which is probably pure phlogiston,8 but I have not yet made one experiment with it.

If volatile alkali, liquid or solid, be exposed to nitrous air, during its effervescence with common air, the vessel is presently filled with beautiful white clouds; and the salt is tinged blue. This explains the constitution of nitrous air, but I have no time for reasoning.9

This experiment appears to great advantage when a vessel, no matter how large, containing the smallest portion of volatile alkali, fluid or solid, is opened in a quantity of nitrous air, for the whole is filled almost instantly with dense white clouds.

Report says that you are about to leave us, at least for a time. If this be true, I shall be very sorry; as it will deprive me of one of the greatest satisfactions that used to make my annual visits to London agreeable. If you should leave England before winter, I should think myself very happy in an opportunity of seeing you before your departure. As I cannot conveniently come to London, I should be peculiarly happy in seeing you and Sir John Pringle, in my new situation; and I flatter myself that I could amuse you with some of my new experiments. If you can oblige me so far, give me a line to acquaint me with your intention, that I may be sure to be at home when you come. I am, Dear Sir, yours sincerely

J Priestley

[Note numbering follows the Franklin Papers source.]

3Priestley was staying with his family in the Wiltshire village, which was on Lord Shelburne’s Bowood estate. Frederick W. Gibbs, Joseph Priestley: Revolutions of the Eighteenth Century (New York, 1967), pp. 86–7.

4The scheme of finding an academic post, which Winthrop had quashed in his letter above to BF, March 4. Priestley’s wording suggests that the comfort of his berth with Lord Shelburne had not dissipated the hope of emigrating.

5Alkaline air is in modern terms gaseous ammonia; acid (which Priestley called marine acid) air is hydrogen chloride gas; and spirit of salt is hydrochloric acid. For “what I have already printed” see above, XIX, 174. Priestley discussed his experiments on ammonia in more detail in Experiments and Observations on Different Kinds of Air (1st ed.; 3 vols., London, 1774–77), III, 294–6; (2nd ed., corrected; 3 vols., London, 1775–84), I, 163–77; and in Experiments and Observations Relating to Various Branches of Natural Philosophy … (2 vols., London, 1779–81), II, 218–24. He was experimenting, as appears later in the letter, with both solid and liquid volatile alkali. The solid was doubtless ammonium carbonate, then called sal volatile; the liquid was, we assume, an aqueous solution of ammonium carbonate called spirit of sal volatile.

6Gaseous ammonia and hydrogen chloride gas, mixed with ordinary air, produce white clouds of ammonium chloride which subsequently form a deposit, then called sal ammoniac, of the same substance.

7For Priestley’s work with nitrous air (nitric oxide) see above, XIX, 201–2.

8Spirit of wine was alcohol; phlogiston, which was basic to the still predominant school of chemical thought, was conceived to be the inflammable element present in all combustible substances.

9Gaseous ammonia, mixed with nitric oxide gas and ordinary air, produces a white cloud of ammonium nitrate. Priestley later continued this line of experimentation and outlined his reasoning about it in the second ed., cited above, of Experiments … on Different Kinds of Air, I, 204–13.

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