Just before you sail’d from New York, you was so good as to inclose to me a Copy of your philosophic Letter to a Friend in Carolina,2 for which please accept my Thanks. The Philosophy of Light and Fire, Heat and Cold has hitherto been a Mystery to me. Even the luminous Emanations of a burning Candle I never could solve in my own Mind. I am half persuaded that Light is an Element different from Fire; which last is an Element which always produces Heat; as does somtimes, tho’ not always the former; unless perhaps there may be such a Connexion between their Operations, that the one excites the other, when perhaps the sole Operation of the Fluid Light would not produce the Sensation of Heat in us. The Glow Worm, Rotten Wood, tainted Flesh, and the Head of a Fish, are all luminous, but not hot: The Flame of a Candle is however luminous and hot. The Fluids of Fire and Light if at rest are imperceptible—their Motions are accompanied with Sensations in us. The Fluid of Heat (to speak improperly) or rather, Fire, could its Motion be excited alone, and by itself only, perhaps might remain invisible: but if such be the Connexions of Nature, that Motion of Fire excites or is generally accompanied with a Motion of the Fluid Light, which is ambient in the same Space, it produces, as we say, Light. But if any Body, metallic or other, cut off the Connexion of mutual Action between the two Elements upon one another, the Motion of one only is excited, the other remains at rest. Iron admits a great degree of Heat before it becomes Luminous. Indeed I have tho’t whether all Motion of the fiery Fluid does not excite a Motion of the luminous one: but that the Surfaces of the Bodies containing Fire already excited or put in Motion in them, that these Surfaces are differently suited to emit and reflect the differently colored Light (the Rays being originally and in themselves differently colored) some Surfaces suited to emit or reflect Rays of one Kind, some of another, some none at all, as the sooted Surfaces of Iron Potts, Kettles &c.—and whether the Action of excited Fire, distending the superficial Pores of Bodies, do not produce all the different Kind of thin Plates, necessary to exhibit the various colored Light—not unlike what is effected in the different Distention of Globules or Bubbles of Water, blown up by a Pipe, or in the variously tinged Clouds in general, or those in particular in which is exhibited the Rainbow.

The Surface of an unpolished Bar of Iron is not suited to reflect Light, I mean copiously, ‘till violent Motion of Fire included in it originally (as all Bodies may be possessed of a mean Quantity) or added injected [sic] into it by the Action of the external motioned Element of Fire—till this violent Motion, by a stated Law, shall have, not only excited the included Light (of which it also possesses a mean Quantity), but distended the superficial Pores, so as to render the Plates which constitute the Surface so thin as readily to emit the excited internal Light already prepared to be transmitted; or reflect the excited external surrounding Light in like manner prepared for Reflection; or lastly, Rays of different orders alternately to reflect and transmit, as in Clouds and particularly the Rainbow. In a burning Coal, or glowing Iron, and glowing melted Metals, the Surfaces are so changed as to reflect copiously, almost all the Light, whence they in this State appear White, or a luminous Glow: This is when there is the greatest Degree of Heat, and consequently when the Pores are most distended, and when the plates are thinnest. When the action of the Fire is less vehement, the Bodies exhibit other Colors—generally Red, or such a Mixture of Rays as make a luminous Whitness: (I mean by Luminous, to distinguish it from the Whitness of Wall, Paper &c.). If I mistake not, the Red Rays refract and reflect the easiest—this may help us to infer the Degree of Distention, or Thinness of Plates. A pretty violent Fermentation of the Fluid Fire (exciting also Light) produced Red; a Ferm[ent]ation still more violent produces a Glowing white. In the last Case Rays of all Kinds are copiously emitted, in the former those of the red only. The glowing White is reflected or emitted from the thinnest Plates.

The sooty Surfaces of Pots and Kettles cant be made to reflect, till the Action of Heat be so Violent, the Heat so intense, as to seperate and throw off the Soot, and perhaps by melting the Iron or Brass cause it [to] be luminous in the Manner above described.

Tho the Action of Fire may produce Motion in Light and render it visible; yet the Action of Light does not always excite Motion in the Fluid Light. As in the Glow Worm &c. The Light in such Bodies is not attended with Heat, is cold. The Rays of the Sun however, when collected by a convex Lens, excite Motion in the Fluid of Fire and renders it perceptible. But the general Law of these mutual Operations we are ignorant of.

Oleaginous Substances, beyond all others, feed and propagate Fire already excited. The violent Collision of dry Sticks of Wood excite it, and indeed feed it; but it soon separates and dissipates the Parts, permitting all their little Moisture to escape and fly off, Leaving only the dry Ashes impregnated with a few Salts too heavy for Evaporation by Adhesion to the rarified ascending air. Oyls feed the Fire much longer. Fire excited, if it has Nothing to operate upon, soon runs into Equilibrium with the common Fluid, and, as we say, dies: but if supplied with dissipable Matter, it acquires greater Force by Action; and being thus furnished with Business for Action, it perseveres ‘till it has accomplished all.

Fire exited set to a Lamp of Oyl, or to a Candle, never leaves it till by its Action it has dissipated the whole. The Motion of Fire communicating or exciting Motion in the surrounding Element of Light, the Light becomes visible in the Flame of the Lamp and Candle. And the Motion of Fire continued by being fed with Oyl or Tallow supplied thro’ the Wick, the Motion of Light in the surrounding Flame or its Accumulation is continued. I should be glad of a more satisfactory Solution.

The undulatory Vibrations of the luminous Fluid, thro’ Action of the Sun in the Center, may produce Light in any part of the System, where are Corpuscles or solid Bodies with Surfaces suited to reflect these Vibrations. As these Vibrations, tho never so contrary and across in their Directions, produce no Confusion: so neither those between the 2 Elements Light and Fire. Newton was of opinion that the Comets, when in the hither Parts of their Trajectories, dispensed beneficent Influences to the System. Perhaps when having passed their Perihelions and ascending charged with great Quantity of solar borrowed Heat and parts of their Bodies in more violent Motion, may communicate an extensive Agitation, on each side the Tract thro’ which they run off, which may serve as necessary Purposes among the Elementary Fluids in the System, as the Tides in the Ocean.

Perhaps there are many Elements, or Kinds of Fluids, in our System, besides Water, Fire, Air, Light—Perhaps Cold may not be the Absence or Cessation of Motion only in Water, Air, Fire &c.; Is it impossible that it should itself be another Element or free passing permeating Fluid? which itself produces the Sensation of Cold in us? and which, in more than a mean Quantity insinuating itself among the other Elements, obstructs their Motion, congeals them? May not volatile Salts, Sulphur and the several Sorts of Aromatics which flow or float freely thro’ Atmospherial Space at least, be so many different Elements? Salt is diffused thro’ the Ocean. They may be confined; Portions may be detatched from the common pervading Mass; Communication may be cutt off: the same may be said of Water, Fire, Air, &c. Ocean Water is as full of Salt as of Light, Air &c. Water and Air obstruct one another: so Air and Salts. If Salts must be supposed sharp pointed; mayn’t Fire with the same propriety? If Salts attract one another, tend to a coalescence; dont a Drop of Water attract Water? If Water floats in Air, dont Nitre? But then you’ll say, dont Birds, Insects, and infinite Animalcula float there too? and are they so many Elements?

There are Elements common to the System, as Light, Heat, Aether: There are Elements peculiar to each planetary Globe, as Water, at least, perhaps the electrical Fluid, and perhaps some animal Fluid which may be the universal Principle of animal Life, the magnetic Effluvia, and aromatic Effluvia which circulate from Vegitables into the ambient Air and again reimbibed by Animals, and Vegitables. And in general all the other different Species of subtle inanimate Matter, so fine as to pass the minute Porosity of Bodies, in perpetual Circulations under the Appointment of general Laws, whether their Particles be globular or angular.

But it matters not as to the Term Element; the Question is, whether there ben’t a Species of fine Subtle Matter that freely permeates Bodies, Fluids &c., which yet of itself when detatchd from any or all other Matter, Applied to our Bodies produce the Sensation of COLD? and whether a certain Mixture of this Matter with Water &c. produces Ice, Snow, and all the Phoenomena of COLD? As salts dissolve cogulated Blood; or rather as Runnet reduces Milk to a Coagulum, so this Element, when in more than mean Quantity, produces Ice &c.3