Benjamin Franklin Papers

To Benjamin Franklin from Joseph Priestley, 27 September 1779

From Joseph Priestley

ALS: American Philosophical Society

Calne Sep. 27. 1779

Dear Sir

Though you are so much engaged in affairs of more consequence, I know it will give you some pleasure to be informed that I have been exceedingly successful in the prosecution of my experiments since the publication of my last volume.8

I have confirmed, explained, and extended my former observations on the purification of the atmosphere by means of vegetation; having first discovered that the green matter I treat of in my last volume is a vegetable substance, and then that other plants that grow wholly in water have the same property, all of them without exception imbibing impure air, and emitting it, as excrementations to them, in a dephlogisticated state. That the source of this pure air is the impure air in the water is evident from all the plants giving only a certain quantity of air, in proportion to the water in which they are confined, and then giving more air in fresh water. I also find that the water before the plants have been confined in it yields impure air, and afterwards pure air.

From these observations I conclude that the reason why my sprigs of mint &c sometimes failed to purify air, was their not being always healthy in a confined state; whereas these waterplants are as much at their ease in my jars as in the open pond.

I have made many other new observations, but they are chiefly of a chymical nature, and not worth while making the subject of a letter; tho, when you see an account of them in my next publication, I flatter myself you will think some of them curious and important.

As the expence of my experiments is necessarily considerable, Dr Fothergill (who, in a very obliging manner, interests himself much in them) has, of his own motion, engaged a few of his friends to contribute about 40£ per Ann to assist me in defraying the expence.9 Indeed, without this assistance I must have desisted altogether. While I had but a small family I suffered myself to be drawn by my success into expences that now appear to have been rather imprudent.

I have been made very happy by the communication of your very ingenious paper on the Aurora Borealis,1 and by several accounts of the good state of your health and spirits. May this long continue— Yours most sincerely

J Priestley

[Note numbering follows the Franklin Papers source.]

8He had completed Experiments and Observations Relating to Various Branches of Natural Philosophy … in March: XXIX, 99–100.

9Fothergill had first offered £100 per year but Priestley declined, fearing the reaction of Lord Shelburne, his employer: Memoirs of Dr. Joseph Priestley, to the Year 1795 … (London, 1806), p. 91.

1XXVIII, 190–200.

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