FIRE, a general name, by which men seem to understand a certain sensation or complex notion of light, heat, burning, melting, &c.
The power of fire is so great, its effects so extensive, and the manner of its acting so wonderful, that some of the wisest nations of old reverenced and worshipped it, as the supreme deity. Some of the chemists also, after they had discovered its surprising operations, suspected it to be an uncreated being: and indeed the most famous of them have acknowledged it as the source of all their knowledge; and hence have professed themselves philosophers by fire, nor thought they could be honoured with a nobler title. Now, amongst all the wonderful properties of fire, there is none more extraordinary than this, that though it is the principal cause of almost all the sensible effects that continually fall under our observation, yet it is itself of so infinitely a subtle nature, that it illudes the most sagacious enquiries, nor ever comes within the cognizance of our senses. Fire is generally divided into three kinds or species, viz. celestial, subterraneous, and culinary.
By celestial fire is principally understood that of the sun, without regard to that of the fixed stars, though this perhaps may be of the same nature.
By subterraneous fire we understand that which manifests itself in fiery eruptions of the earth, volcanoes, or burning mountains; or by any other effects it produces in mines, or the more central parts of the earth.
By culinary fire we mean that employed in all chemical operations, and the common occasions of life.
The sun's heat appears to be the actuating principle, or general instrument of all the operations in the animal, vegetable, atmospherical, marine, and mineral kingdoms.
Fire, considered in itself, seems to exist in the greatest purity and perfection in the celestial regions; at least we are insensible of any considerable smoke it yields:
yields: for the rays of light come to us from the sun, unmixed with any of that gross, feculent, or terrestrial matter, found in culinary and subterranean fires: but, allowing for this difference, the effects of the solar fire appear the same as those of culinary fire.
If we to examine the effects of subterraneous fires, we shall find them the same with those produced by culinary fire. Thus, burnt coals, cinders, and melted minerals, are thrown up by Vesuvius and other burning mountains. Warm nephritical exhalations, natural hot springs, steams, vapours, smoke, &c. are found in several parts of the globe, rising nearly in the same manner as if they were produced by the heat of a furnace. Whence it appears, that subterraneous fires are of the same nature with the culinary.
As men generally affix to the word fire, a complex idea of burning, light, heat, melting, &c. this idea should be analysed, in order to see what parts are essential, and what precarious or arbitrary.
We frequently find the effects of fire produced where no visible fire appeared. Thus the fingers are easily burnt by an iron heated below the degree of ignition, or so as to be no ways visibly red-hot or fiery: whence it follows, that the eye is no judge of fire.
So likewise the touch gives no positive notice of any degree of fire below the natural heat of the body, or any so great as to destroy the organ.
Again, the effects of fire are often produced without any manifest signs of burning, melting, &c. as in evaporations, &c. If this method of exclusion and rejection were pursued to its due length, we should perhaps find no criterion, infallible mark, or characteristic of fire in general, but that of a particular motion struggling among the minute parts of bodies, and tending to throw them off at the surface. If this should prove the case, then such a motion will be the form and essence of fire; and which, being present, makes fire also present: and, when absent, makes fire also absent: whence to produce fire, and produce this motion in bodies, will be one and the same thing.
The great and fundamental difference in respect to the nature of fire is, whether it be originally such, formed thus by the Creator himself at the beginning of things; or whether it be mechanically producible from other bodies, by inducing some alterations in the particles thereof. The former opinion is maintained by Homberg, Boerhaave, the younger Lemery, and s'Gravesande; the latter is chiefly supported by the English philosophers, lord Bacon, Mr Boyle, and Sir Isaac Newton.
Bacon, in the treatise De Forma Calici, deduces, from a great number of particulars, that heat in bodies is no other than motion so and so circumstanced; so that to produce heat in a body, nothing is required but to excite a certain motion in the parts thereof.
Boyle seconds him in an express treatise of the mechanical origin of heat and cold, and maintains the same doctrine with new observations and experiments; as a specimen of which, we shall hear give the two following.
1. In the production of heat, says that able philoso-
pher, there appears nothing on the part either of the agent or patient, but motion and its natural effects. When a smith briskly hammers a piece of iron, the metal thereby becomes exceedingly hot; yet there is nothing to make it so, except the forcible motion of the hammer impressing a vehement and variously determined agitation on the small parts of the iron, which, being a cold body before, grows, by that super-induced commotion of its small parts, hot: first, in a more loose acceptance of the word, with regard to some other bodies, compared with which it was cold before; then sensibly hot, because this agitation surpasses that of the points of our fingers; and in this instance oftentimes the hammer and anvil continue cold after the operation: which shews, that the heat acquired by the iron was not communicated by either of those implements, as heat; but produced in it by a motion, great enough strongly to agitate the parts of so small a body as the piece of iron, without being able to have the like effect upon so much greater masses of metal as the hammer and the anvil: though if the percussions were often and briskly renewed, and the hammer were small, this also might be heated: whence it is not necessary that a body itself be hot to give heat.
2. If a large nail be driven by a hammer into a plank of wood, it will receive several strokes on its head before it grows hot; but when it is once driven to the head, a few strokes suffice to give it a considerable heat; for while, at every blow of the hammer, the nail enters further into the wood, the motion produced is chiefly progressive, and is of the whole nail tending one way; but when that motion ceases, the impulse given by the stroke being unable to drive the nail further on, or break it, must be spent in making a various, vehement, and intestine commotion of the parts among themselves, wherein the nature of heat consists.
Agreeable to this is the opinion of Sir Isaac Newton, who conceives that gross bodies may be converted into light, by the agitation of their particles; and light, again, into gross bodies, by being fixed therein.
On the other hand, M. Homberg, in his Essai du Souffre Principe, holds, that the chemical principle, or element sulphur, which is supposed one of the simple, primary, pre-existent ingredients of all natural bodies, is real fire, and consequently that fire is coeval with bodies.
Dr s'Gravesande goes on much the same principle: fire, according to him, enters the composition of all bodies, is contained in all bodies, and may be separated or procured from all bodies, by rubbing them against each other; and thus putting their fire in motion: but fire, he adds, is by no means generated by such motion.
Mr Lemery, the younger, agrees with these two authors in asserting this absolute and ingenerable nature of fire: but he extends it farther. Not contented to confine it as an element to bodies, he endeavours to shew, that it is equally diffused through all space, and that it is present in all places; in the void spaces between bodies, as well as in the insensible interstices between their parts.
This last sentiment falls in with that of Boerhaave and the celebrated M. Musschenbroek. But notwithstanding what those able philosophers have advanced, it is evident that fire, heat, flame, &c. are only the different modifications of the particles of light, and that the particles of light themselves depend entirely on velocity for their lucific quality; since, by many experiments, we know, that the particles of bodies become lucid, or particles of light, by only producing in them a requisite degree of velocity: thus the particles in a rod of iron, being hammered very nimbly, shine and become red-hot: thus also the violent stroke of the flint against the steel, in striking fire, puts the particles of the steel, which it takes off, into such a motion as causes them to melt, and become red-hot, which makes the sparks of fire produced by each stroke: as, therefore, fire consists in the great velocity of the particles, so it may be communicated from one body in which it is, to another in which it is not, after the same manner that one body in motion will communicate motion to another that has got none.
Fire differs from heat only in this, that heat is a motion in the particles of a body, with a lesser degree of velocity; and fire, a motion with a greater degree of velocity, viz. such as is sufficient to make the particles shine; though we often call such a state as will burn, fire, though it does not actually shine; and we seldom call those lucid bodies fires, which only shine, and do not burn. These are a sort of phosphori, which, though they have no heat, yet seem to owe their lucidity to the motion of their parts.
There seems to be no other difference between fire and flame, than this; that fire consists in a glowing degree of velocity in the parts of a body, while yet subsisting together in the mass; but flame is the same degree of velocity in the particles dissipated and flying off in vapours: or, to use Sir Isaac Newton's expression, flame is nothing else but a red-hot vapour. See FLAME.