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the Royal Society, if you think it worthy a place in their volume; otherwise, I must desire you to return it to the writer. I have another very curious paper containing experiments on the colors seen in the closed eye, after having gazed some time on luminous objects, which is not quite transcribed, but which I will also send to you, if you think it is likely to be acceptable to the Society at this time, but will otherwise let it lie by me another year.
I hope you continue to enjoy your health, and that I shall some time again have the pleasure of seeing you at Staffordshire. I am, dear Sir,
Your affectionate friend,
TO THE MARQUIS DE CONDORCET.
Answers to Questions on Philosophical Subjects.
London, 20 March, 1774.
I am ashamed that my late continued embarras in public affairs should have so long prevented my answering the letter you honored me with, of the 2d of December last.
I transmitted your queries to our Society at Philadelphia, where they will be well considered, and full answers will be sent to you. On my return thither, which I am now preparing for, I shall take care, if not done, to urge the doing it as soon as possible.
In the mean time, I can inform you, as to question first, that, though there is in Pennsylvania abundance of limestone and marble, no flint has yet been found there by the English; yet it is supposed, that flint is to be met with in some part of the country, since heads of arrows made of it by the ancient inhabitants are sometimes found in ploughing the fields. Thus, small sea shells are found intermixed with the substance of rock-stone in some of our highest mountains, and such I think as are not now to be met with on our coasts. Several skeletons, supposed by their tusks to be of elephants, have been found near the Ohio, an account of which may be found in the English Philosophical Transactions.
As to question second, observations have been made in America of the variation of the needle, and, as well as I can remember, it is found to differ a degree in about twenty years.
As to question third; the height of the barometer, by many years' observation, is said to vary between 28.59 and 30-78. The conjectures from these changes are still uncertain.
As to question fourth; the.negroes, who are free, live among the white people, but are generally improvident and poor. I think they are not deficient in natural understanding, but they have not the advantages of education. They make good musicians. As to question fifth; I do not know that any
marks of volcanoes, any lava, or pomice-stone, have been met with in North America. Pit-coal is found in many places, and very good, but little used, there being plenty of wood.
These answers are very short. I hope to procure you such as shall be more full and satisfactory. With great respect, I have the honor to be, Sir, &c.
TO JOHN BAPTIST BECCARIA.
English Translation of Beccaria's Book. — Experiment
to show that Electricity does not pass through a perfect Vacuum.
London, 20 March, 1774. REVEREND AND DEAR SIR, I have received several of your favors lately, relating to the edition of your book in English, which I have put into the hands of the translator, who will observe your directions. The work is now in the press, and goes on pretty fast. I am much obliged by your kind assistance in procuring the impressions from the plates. They are not yet arrived here; but the money, which I find by a note from you to Dr. Priestley amounts to one hundred and forty-three livres of Piedmont, will be paid by the bookseller, Mr. Nourse, in my absence, to any person you may order to receive it.
Mr. Walsh, the same ingenious member of our Society who went to France to make experiments on the Torpedo, has lately hit on a new discovery in electricity, which surprises us a little. You know that finding air, made rarer by the pump or by heat, gave less obstruction to the passage of electricity, than when in its denser state, we were apt to think a perfect vacuum would give it no resistance at all. But he, having by boiling the mercury made a perfect vacuum in a long bent Torricellian tube, has found that vacuum absolutely to resist the passage of the electric fluid during two or three days, or till some quantity of air, the smallest imaginable, is admitted into it. This, if verified by future experinients, may afford some new light to the doctrine
* The remainder of the letter is lost. — Editor.
Effect of Vegetation on Noxious Air.
That the vegetable creation should restore the air which is spoiled by the animal part of it, looks like a rational system, and seems to be of a piece with the rest. Thus fire purifies water all the world over. It purifies it by distillation, when it raises it in vapors, and lets it fall in rain ; and farther still by filtration, when, keeping it fluid, it suffers that rain to percolate the earth. We knew before, that putrid animal substances were converted into sweet vegetables, when mixed with the earth, and applied as manure ; and now it seems, that the same putrid substances, mixed with the air, have a similar effect. The strong thriving state of your mint, in putrid air, seems to show, that the air is mended by taking something from it, and not by adding to it.
I hope this will give some check to the rage of destroying trees that grow near houses, which has accompanied our late improvements in gardening, from an opinion of their being unwholesome. certain, from long observation, that there is nothing unhealthy in the air of woods; for we Americans have everywhere our country habitations in the midst of woods, and no people on earth enjoy better health, or are more prolific.
* This extract is taken from Priestley's Experiments on Air, (Vol. I. p. 94,) 3d Edition. The author introduces it with the following remark. “Dr. Franklin, who, as I have already observed, saw some of my plants in a very flourishing state, in noxious air, was pleased to express very great satisfaction with the result of the experiments. In answer to the letter in which I informed him of it, he says,” &c. — EDITOR.
TO JOSEPH PRIESTLEY.*
On the Inflammability of the Surface of certain Rivers
Craven Street, 10 April, 1774. DEAR SIR, In compliance with your request, I have endeavoured to recollect the circumstances of the American experiments I formerly mentioned to you, of raising a flame on the surface of some waters there.
When I passed through New Jersey in 1764, I heard it several times mentioned, that, by applying a lighted candle near the surface of some of their rivers, a sudden flame would catch and spread on the water, continuing to burn for near half a minute. But the accounts I received were so imperfect, that I could form no guess at the cause of such an effect, and rather doubted the truth of it. I had no opportunity of seeing the experiment; but, calling to see a friend who happened to be just returning home from making it himself, I learned from him the manner of it; which was to choose a shallow place, where the bottom could
a be reached by a walking-stick, and was muddy; the mud was first to be stirred with the stick, and, when a number of small bubbles began to arise from it, the candle was applied. The flame was so sudden and so strong, that it catched his ruffle and spoiled it, as I saw. New Jersey having many pine-trees in many parts of it, I then imagined that something like a volatile oil of turpentine might be mixed with the waters from a pine. swamp, but this supposition did not quite satisfy me.
* See Priestley's Experiments on Air, (Vol. I. p. 321, 3d Edition.