Gilmanton, N.H. Apr. 7. 1802.
I some weeks since had the pleasure of seeing an advertisement issued by your order, for the purpose of encouraging discoveries relative to Light & Heat.—Such a laudable stimulus to the promotion of Science from so eminent a Patron of useful Knowledge, must, I think, meet the approbation it merits, and contribute to useful discoveries, not only by exciting the emulation of the aspiring & enterprising; but also, “if it were possible,” by even stirring up the dormant faculties of the unambitious and indolent, for the sake of obtaining the Prize.
My object in writing this is neither riches nor honor;—neither have I a prospect of communicating any thing new on the subject of Light or Heat. But, as I have ever been delighted with scientific studies, yet without having those advantages which were requisite for obtaining that share of knowledge which I would gladly possess, I was encouraged by your noble and generous proposal to solicit your opinion, or answer, to the following queries relative to Light; provided you, Sir, shall deem such a condescention not repugnant to the rules and dignity of the respectable society over which you preside.
As Light is something, which eminent authors define different ways, it appears to me, that, previous to expatiating largely on its effects, it is necessary to investigate, as far as possible, its essence and nature,—This, according to the new method of philosophizing, is no difficult task; but my knowledge is so circumscribed that I cannot possibly see how Light can be what many will have it to be.
1st: Light is said to be a substance; but if that is the fact, why does it not, according to an established law of nature, gravitate towards the Earth and other planets, in its passage from the sun, and so form parabolic curves, like other projectiles?—But the rays of light are known to proceed in right lines.
2d: It has been found by experience, that the rays of light proceed equally from all sides of a luminous body, but if they were material, how could the projectile force with which they are impelled, impel the particles as high above the luminary, as tho some force and that of gravity united would depress them below it?
3rd: It is maintained that light passes from the sun to the Earth in about 8 minutes, without exceptions, at a mean rate: But if light were matter, yet lighter than air, why would it not remain suspended on air of the same specific gravity, and consequently never reach the Earth?—Or, if it were specifically heavier than air, why does not the mercury in the Barometer rise to more than double the height in a dry, clear day, than in the night, when more than half the weight of the atmosphere must be deducted, on account of the absence of Light?
4th: If light be material, why does it not penetrate cork easier than glass? When the density of the former is but 240, and the latter 2666?
5. Mr: Gravesande affirms that heat, in a hot body, is nothing but motion, and if that motion proceeds in right lines, it gives us the idea of light: but if in an irregular manner, only of heat.—Why, then, if a nail be driven by a force perpendicular to its diameter, till it is very much heated, does not its reaction in an uniform right line, produce light?
6. Sir Isaac Newton says "The rays of light are hard bodies." And Dr. Niewentijt computes that in one second of time there flow out of a burning Candle, 418,660,000,000,000,000,000,000,000,000,000,000,000,000,000 particles of light.—Now suppose an eye be placed 6 inches from the center of a burning candle, and that distance to be the radius of a globe of rays, then the surface of such a globe would contain 113 1/7 square inches;—Again, suppose the visible part of an human eye to be one square inch, and the flame of the Candle perfectly globular, and the eye will be acted upon by 370,495,575,221,238,938,053,097,345,132,743,362, 831,185,84 of those "Hard bodies" projected with a velocity of 164000 miles pr. second.—Why do not those accumulated particles falling on our eyes with such a prodigious momentum, immediately destroy them, even if those particles are of no greater magnitude than points?
7. Mr. Martin, Ferguson, Dr. Keill and others, are of opinion that if our Earth had no atmosphere, the Heavens would appear dark in the day time: But if light were substantially emitted from the sun to the Earth why, if we could live without air, and turned our back to the Sun, should we not see the Earth enlightened as usual?
8. In trying to discover the Gravity of Light, I adopted the following simple methods. I firstly took two pieces of rotten wood, from the same trunk; one piece of which, when placed in the dark, continually emitted light, like phosphorous, and the other did not. Then suspending a small pair of scales in a darkened room, that I might better observe, the nature of the light emitted, I put the shining piece of wood into one scale, and balanced it by a piece that emitted no light in the other. And after they had been suspended for several days, the equibilibrium was not destroyed, but remained exactly as at the first.—
I next put into one scale a piece of white cloth, and suspended the scales in equilibrio by a piece of black cloth in the other: and adjusting them in such a manner, that the sun, then at his greatest altitude, might shine into each scale. Then collecting the rays by means of a Lens placed its focal distance from the scale, I caused the rays to fall upon the black cloth for the space of 1/2 an hour: but though the black cloth absorbed, and the white one reflected the rays, yet the scales remained in equilibrio, tho’ the whole process.
Now if light be a material substance, why did not the increase and diminution of gravity caused by the absorption of the rays in the latter case, & by their being emitted in the former, impair or else totally destroy the equilibrium?
9. If light is the original source of all colour, and no color can exist without it, how did subterraneous substances receive their colours? Or why do we find those plants which have sprung up and grown in darkness, do appear of the same color as the rest of their kind which have grown in the light, upon the first admission of light?—Will any say that they receive their colour instantaneously, and that light only gives them their colours? For since the atmosphere may be changed to any colour you please, by placing different colors at certain angles with the incident rays of light, why may we not as well say that the atmosphere receives its particular hue from colors created by the Deity, as that every substance receives its colouring in a secondary manner from the rays of [light]?
From your friend and humble servant
MHi: Adams Papers.