In this series of speculations, Franklin tried to combine into one theory his long-held beliefs about light, heat, and fire. The concept he used to link these phenomena—the subtle fluid—had become an integral part of physics in the second half of the eighteenth century. Subtle, or “imponderable,” fluids possessed and could pass on measurable physical properties, but were themselves invisible, immaterial, and weightless, and they did not cause any changes in mass in the objects they acted on. The effects of subtle fluids were believed to be observable and quantifiable in the workings of electricity, heat, gravity, light, and magnetism.1

Franklin had sketched out the basic ideas for this paper in a letter to Benjamin Vaughan of April 29 (above), written just as he and other court-appointed scientists were beginning to investigate the so-called magnetic fluid that Franz Mesmer termed animal magnetism. By June 25, when this piece was finished, he and his fellow commissioners had proved that Mesmer’s animal magnetism did not exist.2 Their conclusions did not negate the concept of subtle fluids, however, and Franklin clearly supposed that his ideas on the subject would be of general interest.

Franklin had his grandson Benny Bache make copies of this piece in both English and French. One of the English copies was sent to Benjamin Vaughan, who in turn made his own copy.3

As for the French translation (the draft of which does not survive), Benny made an initial copy which Franklin corrected, and that text was then copied by Morellet’s secretary, Poullard. Poullard’s copy was then reviewed by Franklin and Cabanis, who refined the wording. A fair copy of that final French translation was made by the abbé de la Roche. From there, the trail grows cold; if the piece sparked a conversation within the Parisian scientific community, it does not appear to have resulted in a publication.

Before leaving France, Franklin sent this essay to Jan Ingenhousz, who had requested it.4 Once home, he offered it and other scientific papers to James Bowdoin for the American Academy of Arts and Sciences.5 Receiving neither an acceptance nor a rejection, in 1788 he sent the papers to the American Philosophical Society. By that time he had made a few changes to his original draft (noted in the text below), including the addition of a final sentence. He also squeezed in the following line at the top of the first page: “For the Consideration of my Dear Friend David Rittenhouse Esqr.”6 The paper was read to the American Philosophical Society on June 20, 1788. When it appeared in the society’s next volume of Transactions (1793), it bore the title “A new and curious Theory of Light and Heat; in a letter from Dr. B. Franklin to David Rittenhouse, Esq.”7

Universal Space, as far as we know of it, seems to be fill’d with a subtil Fluid, whose Motion, or Vibration; is called Light.8

This Fluid may possibly be the same with that which being attracted by and entring into other more solid Matter, dilates the Substance, by separating the constituent Particles, and so rendering some Solids fluid, and maintaining the Fluidity of others: of which Fluid when our Bodies are totally depriv’d, they are said to be frozen; when they have a proper Quantity, they are in Health and fit to perform all their Functions; it is then called natural Heat; when too much it is called Fever; and when forc’d into the Body in too great a Quantity from without, it gives Pain by separating and destroying the Flesh & is then called burning,—and the Fluid so entring & acting is called Fire.9

While organiz’d Bodies animal or vegetable are augmenting in Growth, or are supplying their continual Waste, is not this done by attracting and consolidating this Fluid, called Fire, so as to form of it a Part of their Substance; and is it not a Separation of the Parts of such Substance which dissolving its solid State, sets that subtil Fluid at Liberty, when it again makes its appearance as Fire.

For the Power of Man relative to Matter, seems limited to the dividing it, or to mixing the various kinds of it, or changing its Form and Appearance by different Compositions of it, but does not extend to the making or creating of new Matter, or annihilating the old: Thus if Fire be an original Element or kind of Matter its Quantity is fix’d and permanent in the Universe.1 We cannot destroy any Part of it, or make addition to it. We can only separate it from that which confines it & so set it at Liberty, as when we put Wood in a Situation to be burnt; or transfer it from one Solid to another, as when we make Lime by burning Stone, a Part of the Fire dislodg’d from the Wood being left in the Stone.

May not this Fluid when at Liberty be capable of penetrating & entring into all Bodies organiz’d or not: quitting easily in totality those not organiz’d, and quitting easily in part those which are; the part assum’d and fix’d remaining till the Body is dissolv’d.—

Is it not this Fluid which keeps asunder the Particles of Air, permitting them to approach or separating them more in Proportion as its Quantity is diminish’d or augmented?

Is it not the greater Gravity of the Particles of Air, which forces the Particles of this Fluid to mount with the Matter to which it is attach’d as Smoke or Vapour?

Does it not seem to have a great Affinity with Water, since it will quit a Solid to unite with that Fluid, & go off with it in Vapour, leaving the Solid cold to the Touch, & the degree measurable by the Thermometer?—

The Vapour rises attach’d to this Fluid but at a certain height they separate, and the Vapour descends in Rain retaining but little of it, in Snow or Hail less. What becomes of that Fluid? Does it rise above our Atmosphere and mix with2 the universal Mass of the same kind?

Or does a Spherical Stratum of it, denser or less mix’d with Air, attracted by this Globe, & repell’d or push’d up only to a certain height from its Surface by the greater Weight of Air remain there, surrounding the Globe and proceeding with it round the Sun.3

In such case, as there may be a Continuity or Communication of this Fluid thro’ the Air quite down to the Earth, is it not by the Vibrations given to it by the Sun that Light appears to us; and may it not be, that every one of the infinitely small Vibrations, striking common Matter with a certain Force, enters its Substance, is held there by Attraction, and augmented by succeeding Vibrations, till the Matter has receiv’d as much as their Force can drive into it?—

Is it not thus that the Surface of this Globe is continually heated by such repeated Vibrations in the Day, & cooled by the Escape of the Heat when those Vibrations are discontinu’d in the Night, or intercepted & reflected by Clouds?

Is it not thus that Fire is amass’d in and makes the greatest Part of the Substance of combustible Bodies?

Perhaps when this Globe was first form’d and its original Particles took their Place at certain Distances from the Centre in proportion to their greater or less Gravity the Fluid Fire attracted towards that Center might in great Part be oblig’d as lightest to take place above the rest, and thus form the Sphere of Fire above suppos’d Which would afterwards be continually diminishing by the Substance it afforded to organiz’d Bodies, and the Quantity restor’d to it again by the Burning or other Separating of the Parts of those Bodies?

Is not the natural Heat of Animals thus produc’d, by separating in digestion the Parts of Food, and setting their Fire at Liberty?—

Is it not this Sphere of Fire which kindles the wandring Globes that sometimes pass thro’ it in our Course round the Sun, have their Surface kindled by it, and burst when their included Air is greatly rarified by the Heat on their burning Surfaces?—4

May it not have been from such Considerations that the antient Philosophers suppos’d a Sphere of Fire to exist above the Air of our Atmosphere?5