Fire is so subtile an agent in nature, that we can reason little concerning it, except from experiment; and we are even at a loss from thence to determine, in many cases, either its absolute quantity or real presence. If, with the generality of philosophers, we assume heat as its characteristic property, and define fire to be that which warms or heat bodies, we cannot avoid seeing, at once, the ambiguity of this criterion: it is as precarious as the perceptions of different men at the same time, or of the same man at different times, in summer and in winter, in a fever and in health. The light of the moon, when collected into the focus of a large burning glass, is found to be about one thousand times less dense than the direct rays of the sun: hence it is, that it excites no motion in the mercury of the most sensible thermometer. But from the brightness of the image in the focus, as well as from the luminous appearance of rotten wood, putrid fish, and other phosphorescent bodies, some philosophers have inferred, that wherever there is light there is fire: but as the converse of this proposition is not true, since fire often exists in large quantities, as in boiling fluids, in metals moderately heated, &c. without Light; this cannot be the distinguishing mark of the presence of fire. The dilatation which fire occasions in all bodies, whether solid or fluid, hard or soft, light or heavy, may be esteemed the most certain proof of its presence and agency. This property serves admirably to mark its degrees and minute variations within certain limits, but not to ascertain either its presence or its quantity in extreme cases, unless we know the real magnitudes of bodies totally destitute of it. However, admitting this phenomenon as the most certain indication of the existence of fire, it may be accounted for in the following manner:
From the 14th section of Sir Isaac Newton's Principia, we learn, that the motions of small bodies, when attracted perpendicularly towards any surface, according to any law, are similar to the motions of the rays of light, with respect to the fundamental properties of Inflection, Reflection, and Refraction: from hence chiefly, as well as from other arguments, we infer, that rays of light are small corpuscles, admitted from shining bodies, and moving with uniform velocities in uniform mediums; but with variable velocities in mediums of variable densities. This being admitted, it will follow, that in whatever quantity the rays of light are made to move in a medium of an uniform density, they will not agitate the particles, or produce any augmentation of bulk in that medium. If the atmosphere was reduced to medium of an uniform density, surrounding the earth every where to the height of five miles, it would be expanded in bulk, or warmed only at its outward and inward surface. The sun's rays, by coming out of a vacuum into a denser medium, would be attracted by the particles composing that medium; and, since all attraction is mutual, they would excite a motion, an expansion, and heat, at the outward surface where they entered: from thence they would proceed uniformly, without producing any effect, till they came to the inner surface of the atmosphere contiguous to the surface of the earth, where they would undergo another acceleration of velocity, and would excite another degree of motion, another degree of expansion or heat. Such an atmosphere would be the coldest in the middle, the heat decreasing from each surface. We may, perhaps, from what has been said, conceive, in some measure, how bodies are expanded, heated, and volatilized, by the agency of the particles of light. These particles act upon the minute constituent parts of bodies, not by impact, but at some indefinitely small distance; they attract, and are attracted; and in being reflected, or refracted, they excite a vibratory motion in the component particles. This motion increases the distance between the particles; an increase of the distance between the constituent parts of any body, is an augmentation of bulk, an expansion in every dimension,—the most certain characteristic of fire. This expansion, which is the beginning of a disunion of the parts, being increased by the increasing magnitude of the vibrations proceeding from the continued agency of the light, it may easily be apprehended, that the particles will at length vibrate beyond their sphere of mutual attraction, and thus the texture of the body will be altered or destroyed: from solid it may become fluid, as in melted gold; or from being fluid, it may be dispersed in vapor, as in boiling water.
According to this theory, we must infer that the constituent parts of all bodies are in perpetual motion. The temperature of the atmosphere is different in different latitudes, and it changes almost every instant, in the same. The temperature of bodies is ever proportionable to that of the surrounding atmosphere, and from thence it must be perpetually varying. The bulk of every body is proportionable to its temperature, and must therefore be subject to a perpetual vicissitude. Now the body will be in an expanded, in the next instant, its heat happening to be diminished, it will be in a contracted state; which variation of dimensions cannot be effected without a perpetual vibratory motion of its constituent parts.
It being established then, that the rays of the sun, even in their most condensed state, as in the focus of a burning speculum, do not otherwise produce heat than as they excite a motion more or less violent amongst the constituent parts of bodies; and the effects of culinary fire, of that produced by friction, or by the impact of hard bodies, being similar to those produced by the agency of the sun's light, it may be conjectured, that they are produced after a similar manner; and that fire is nothing distinct from the parts of bodies put into motion by various causes, as the impulse of light, friction, percussion, putrefaction, attraction of cohesion, &c. and consequently that it may be mechanically produced, altered, or destroyed in all bodies, with greater or less facility, according as the parts of the body are more or less disposed for motion.
This conclusion seems to be consonant with the principles of the received philosophy. Newton in his 5th quaere annexed to his Optics, asks, Do not bodies and light act mutually upon one another? that is to say, bodies upon light in emitting, reflecting, refracting and inflecting it; light upon bodies for heating them, and putting their parts into a vibratory motion, wherein heat consists?
There are various other opinions concerning the nature of fire and its method of action, which, though different from what has been offered, are not less probable: I will content myself with mentioning two more.
Boerhaave thinks that fire is a fluid of a nature peculiar to itself; that it was created such as it is, and cannot be altered in its nature or properties, destroyed or produced; that naturally exists in equal quantities in all places; that it is wholly imperceptible to our senses, and only discoverable by such effects as it produces when by various causes it is for a time collected into a less space than what, from its tendency to an universal and equable diffusion, it would otherwise occupy. All the bodies which are situated in the immensity of space, may, according to this opinion, be divided into fire expanding all other bodies, and into all the other bodies which are not fire, but resist its action. The matter of this fire is not supposed to be derived from the sun in any wise; the solar rays, whether directed or reflected, are of use only as they impel the particles of fire in parallel directions: that parallelism being destroyed, by intercepting the solar rays, the fire instantly resumes its natural state of uniform diffusion. Consistent with this explication, which attributes heat to the matter of fire, when driven in parallel directions, a much greater must be given it, when the quantity so collected, is amassed into a focus; and yet the focus of the largest speculum does not heat the air, or medium in which it is formed, but only bodies of densities different from that medium.
The author of the Lettres Physiques is of opinion, that the solar rays are the principal cause of heat; but that they only heat such bodies as do not allow them a free passage. In this remark he is agreed with Newton: but then he differs totally from him, as well as from Boerhaave, concerning the nature of the rays of the sun. He does not admit the emanation of any luminous corpuscles from the sun, or other self-shining substances, but supposes all space to be filled with an ether of great elasticity and small density, and that light consists in the vibrations of this ether, as sound consists in the vibrations of the air; the particles of the one medium exciting, by impulse upon the organ of vision, the idea we call light; the particles of the other medium exciting, by impulse on the organ of hearing, the idea we call sound. But as a bell will not of itself begin those vibrations by which the air is put in motion, nor continue with equal intensity the vibrations, when once excited, without the concurrence of some mechanical cause; so neither will the sun either begin or continue his vibrations, by which the supposed ether is put in motion, without a similar mechanical agency. In ascending from effects to causes we must ever arrive, upon whatever hypothesis we proceed, at some first cause, which does not admit an explanation from mechanical principles; this is evidently the case in the present inquiry. Upon Newton's supposition, the cause by which the particles of light, and the corpuscles constituting other bodies, are mutually attracted and repelled, is uncertain. The reason of the uniform diffusion of fire, of its vibration, and repercussion, as stated in Boerhaave's opinion, is equally inexplicable; and in the last mentioned hypothesis, we may add to the other difficulties attending the supposition of an universal ether, the want of a first mover to make the sun vibrate. These are the opinions most worthy of notice, concerning elementary fire; and of these it may be said, as Cicero remarked of the opinions of philosophers concerning the nature of the soul—harum sententiarum quae vera sit, Deus aliquis viderit, quae verisimillima, magna questio est.[1,2]
But besides this elementary fire, which chemists conceive to be every where uniformly diffused, they are of opinion that fire enters, in different proportions, into the composition of all vegetables and animals, and most minerals; and in that condensed, compacted, fixed state, it has been denominated the phlogiston. Of itself, in its natural state of uncombined expansion, fire is not esteemed capable of shining, or burning: when chemically conjoined with the other principles of bodies, it is that alone which conceives and continues those motions by which bodies are made to shine, to burn, to consume away. All bodies are more or less susceptible of combustion, according to the quantity of this principle which enters into their composition, or the degree of force with which it adheres to them. In the act of burning, and it may very probably be during the fermentation, and putrefaction, and chemical solutions of various bodies, it recovers its fluidity, is expanded and dispersed into the air, or combined anew with such substances as it has an attraction to. Notwithstanding all that perhaps can be said upon the subject, I am sensible the reader will be still ready to ask—what is phlogiston? You do not surely expect that chemistry should be able to present you with a handful of phlogiston, separated from an inflammable body; you may just as reasonably demand a handful of magnetism, gravity, or electricity, to be extracted from a magnetic, weighty, or electric body. There are powers in nature which cannot otherwise become the objects of sense, than by the effects they produce; and of this kind is phlogiston. But the following experiments will tend to render this perplexed subject somewhat more clear.
If you take a piece of sulphur and set it on fire, it will burn intirely away, without leaving any ashes, or yielding any soot. During the burning of the sulphur, a copious vapor, powerfully affecting the organs of sight and smell, and the actions of the lungs, is dispersed. Means have been invented for collecting this vapor, and it is found to be a very strong acid. The acid thus procured from the burning of the sulphur, is incapable of being either burned by itself, or of contributing towards the support of fire in other bodies: the sulphur from which it was procured was capable of both: there is a remarkable difference then, between the acid procured from the sulphur, and the sulphur itself. The acid cannot be the only constituent part of sulphur; it is evident that something else must have entered into its composition, by which it was rendered capable of combustion. This something is, from its most remarkable property, that of rendering a body combustible, properly enough denominated the food of fire, the inflammable principle, the phlogiston.
From this analysis we may conclude, that the constituent parts of sulphur are two;—an inflammable principle, which is dispersed in the act of combustion, and an acid. The proportion of these parts has been ascertained; and it is found, that in any mass of sulphur, the weight of the inflammable principle is to that of the acid in the proportion of 3 to 50.
If you burn charcoal in the open air, and hold a glass over its flame, you will perceive that it burns without emitting either any watery vapour or sooty impurity; and nothing will remain, from a large portion of charcoal, but a small portion of white ashes, which are incapable of any further combustion. The principle effecting the combustion of the charcoal, and dispersed by the act of combustion, is the phlogiston.
If you set spirits of wine on fire, they will, if pure, burn intirely away; they differ from charcoal in this, that they emit a vapour: but they leave no residuum. You may by proper vessels collect the vapour of burning spirits, and you will find it to be an insipid water, incapable of combustion. The principle effecting the combustion of the spirits of wine, and dispersed by the act of combustion, is the phlogiston.
Some metallic substancesburn, when sufficiently heated, with a flame more bright than that of spirits of wine, or charcoal; others burn or smother away like rotten wood; and most of them, when they have been kept in the open air in a proper degree of heat, lose their metallic appearance, and are converted into earth. Thus red lead is the earth procured from the burning of lead; and putty, such as the polishers of glass and marble use, is the earth procured from tin. The principle effecting the combustion of metallic substances, and dispersed in the act of combustion, is the phlogiston.
The acid of the sulphur; the ashes of the charcoal; the water of the spirits of wine; the earths of metallic substances, are utterly incapable of combustion: their respective differences from sulphur, charcoal, spirits of wine, and metallic substances, with respect not only to inflammability, but to smell, colour, consistency, and other properties, are attributed to the phlogiston which is dispersed during the combustion of each of them.
This inflammable principle, or phlogiston, is not one thing in animals, another in vegetables, another in minerals, it is absolutely the same in them all; just as water which enters into the composition of flesh, wood, coal, is still water, though its existence and homogeneity be rendered more doubtful in some substances than in others. This identity of phlogiston may be proved from a variety of decisive experiments; I will select a few which may at the same time confirm what has been advanced concerning the constituent parts of sulphur.
From the analysis or decomposition of sulphur effected by burning, we have concluded, that the constituent parts of sulphur are two—an acid which may be collected, and an inflammable principle which is dispersed. If the reader has yet acquired any real taste for chemical truths, he will wish to see this analysis confirmed by synthesis; that is, in common language, he will wish to see sulphur actually made, by combining its acid with an inflammable principle. It seldom happens that chemists can reproduce the original bodies, though they combine together all the principles into which they have analyzed them; because not only the number and proportions of the principles, but the order also of their arrangement must be observed before that can be effected: in the instance, however, before us, the reproduction of the original substance will be found complete.
As the inflammable principle cannot be obtained in a palpable form separate from all other bodies, the only method by which we can attempt to unite it with the acid of sulphur, must be by presenting to that acid some substance in which it is contained. Charcoal is such a substance, and by distilling powdered charcoal and the acid of sulphur together, we can procure a true yellow sulphur, in no wise to be distinguished from common sulphur. This sulphur is formed from the union of the acid with the phlogiston of the charcoal; and the charcoal may by this means be so entirely robbed of its phlogiston, that it will be reduced to ashes, as if it had been burned. Animal substances reduced to the state of a black coal, will, by being treated in the same way, yield sulphur.
Spirits of wine, we have said, consist of phlogiston united with water; and if we distill a mixture of spirits of wine and the acid of sulphur, we shall towards the end of the operation obtain a pure sulphur.
Oil of turpentine is very inflammable, and consequently abounds with the principle which has been denominated phlogiston; and from a distillation of acid of sulphur with oil of turpentine, a sulphur may be procured.
But one of the shortest and most obvious ways of illustrating both the composition of sulphur and the phlogiston of metallic substances, is the following.—Upon melted lead pour the acid of sulphur; collect the vapor which will arise, by holding a very large glass or other vessel over the melted lead, and you will, as soon as the vapor is condensed, observe several filaments of sulphur sticking to the sides of the glass.—When lead is in a state of strong fusion, its phlogiston is in a state of dispersion: the acid of sulphur instantaneously unites itself with this phlogiston, and forms sulphur. It is probable, that sulphur might be procured by the same means from a variety of other bodies, when in a state of actual combustion.
I will in this place, by way of further illustration of the term phlogiston, add a word or two concerning the necessity of its union with a metallic earth in order to constitute a metal.
Lead, it has been observed, when melted in a strong fire, burns away like rotten wood; all its properties as a metal are destroyed, and it is reduced to ashes. If you expose the ashes of lead to a strong fire, they will melt; but the melted substance will not be a metal, it will be a yellow or orange-coloured glass. If you pound this glass, and mix it with charcoal dust, or if you mix the ashes of the lead with charcoal dust, and expose either mixture to a melting heat, you will obtain, not a glass, but a metal, in weight, colour, consistency, and every other property the same as lead. This operation, by which a metallic earth is restored to its metallic form, is called Reduction. The ashes of lead melted without charcoal become glass; the ashes of lead melted with charcoal become a metal; the charcoal then must have communicated something to the ashes of lead, by which they are changed from a glass to a metal. Charcoal consists of but of two things, of ashes, and of phlogiston; the ashes of charcoal, though united with the ashes of lead, would only produce glass; it must therefore be the other constituent part of charcoal, or phlogiston, which is communicated to the ashes of lead, and by an union with which the ashes are restored to their metallic form. The ashes of lead can never be reduced to their metallic form, without their being united with some matter containing phlogiston; and they may be reduced to their metallic form, by being united with any substance containing phlogiston in a proper state, whether that substance be derived from the animal, vegetable, or mineral kingdom (for tallow, or iron filings may be substituted with success in the room of charcoal, in the experiment of reducing the ashes of lead); and thence we conclude, not only that phlogiston is a necessary part of a metal, but that phlogiston has an identity belonging to it, from whatever substance in nature it be extracted. And this assertion still becomes more general, if we may believe that metallic ashes have been reduced to their metallic form, both by the solar rays and the electrical fire.
 The reader who is desirous of making a deeper inquiry into this matter, may consult a very ingenious tract, entitled, Experiments and Observations on animal Heat, and the Inflammation of combustible Bodies, by Dr. Crawford; and Mr. Scheele’s Experiments on Air and Fire, translated from the German by Dr. Forster, and illustrated with judicious notes by Mr. Kirwan; and a late work of Wallerius’, entitled Mediationes de Orig. Mund. May not the common degree of heat which arises from the mixture of different quantities of the same fluid heated to different degrees, be investigated by the same rule, by which the common velocity of hard or non-elastic bodies after their impact in the same direction is calculated, putting the momentum of heat to be equal to its degree multiplied into the quantity of heated matter? --original note
 The experiment from which this proportion is derived, is said to have been made by M. Brandt with great accuracy about the year 1756. Spielm. Chem. p. 111. and Chem. Dict.—Newmann from a similar experiment infers, that in 16 ounces of sulphur, there are upawards of 15 3/4 ounces of pure acid, and not quite 1/4 of an ounce of the inflammable principle. Newm. Chem. p. 168. --original note