.

that the substance of these is one and the same; and therefore, I shall speak of each in

- discriminately, under the general name of fire, passing from one to the other as the subject shall naturally lead me.

Fire is a Corporeal Substance.

There having been different opinions of philosophers concerning fire; some contending, that it is incorporeal, and others, that it is nothing of itself, but only the effect of motion in the solid parts of bodies; it will be proper to shew, in the first place, that fire is material, and hath actual extension as a body: which will be plain from the following considerations:

1. That fire can drive out other matter from any given space; and certainly that which can expel other bodies, and take the place of them, must itself be body. If the ball of a thermometrical tube is filled with air, spirits, or mercury, fire applied underneath it will expel them all in their turns; which it cannot do, but in virtue of its own proper extension; and if it is extended, it is a bodily substance.

2. That fire may be seen to pass through

a liquor

a liquor in bubbles, like air, and is therefore material as the air itself is. When a glass phial, three parts full of pure water, is placed upon live coals of wood, so as to acquire its heat by degrees lest the glass break; when it is beginning to boil, a fine transparent matter is seen to shoot in subtile streams through the bottom of the vessel, expanding itself as it ascends through the water; and, as the boiling increases, this matter enters with greater velocity, and with a greater expansive force; impelling the water before it by a true corporeal percussion, and with a power far superior to the pressure of the atmosphere, as we shall have occasion to shew in another place.

3. A fluid, subject to like laws with the elastic air, must be material as the air is. Fire, in common with air, is subject to be confined by an incumbent an incumbent pressure, and released when that pressure is withdrawn. Fire would make water boil much sooner, if it were not resisted by the pressure of the atmosphere upon its surface; and therefore it boils with a very low degree of heat in the vacuum of an air-pump. Fire evaporates from an heated liquor more slowly when counteracted by the pressure of the air,

which hinders it from being freely expanded. If two equal vessels of water, equally heated, are set to cool, one of them under the exhausted receiver of an air pump, and the other in the open air; the water under the receiver will be found to cool much faster than the other. Therefore fire is confined by an incumbent pressure, and evaporates freely when there is less resistance. The same is evinced by many experiments in electricity, where the electric fire is dissipated, unless it is resisted by the pressure of the air on the surface of electrised bodies; and hence its force is always greatest when the barometer is highest.

4. It is farther evident, that fire is a body, because light (which is the same in substance) is subject to this common law of projected bodies, that the angle in which it is reflected is equal to the angle of its incidence, which cannot possibly follow unless it is understood to be a body impinging upon a surface. That it is a body of extreme and inconceivable subtilty may be proved from the consideration of this particular case; that if light falls upon a surface, in a direction perpendicular to the surface, it is reflected from it again in the same perpendi

cular.

cular. It has been argued from this case, that light has not that impenetrability which is essential to matter, being thus returned by reflexion as it were into itself, and lost in its own substance. But it hath already been observed, that such is the nature of fluids, especially of this the most subtile of all fluids, that they can move in a direction contrary to themselves, by the sliding of the parts beside each other: and it is farther to be observed, that light so reflected upon itself becomes so much the more intense; which shews that there is a fresh accession of matter to the column by the reflexion.

5. Boerhaave brings it as an argument that fire is a true corporeal substance, because the rays of the sun, from a very powerful burning glass, directed to the extremity of a magnetic needle, gave motion to it as a stroke or blast would have done. But if we want any evidence that fire can affect bodies with a true corporeal percussion, we have nothing to do but to feel the shock of it in electricity; the power of which is so effectual upon the muscles, as to leave no possible doubt in the understanding. And here, in the experiments of electricity, we constantly find that the larger column has

the

the greater force, according to the law which obtains in other matter; from the smallest thread of an explosion, up to the column of fire in lightning; the dimensions of which far exceed all that art can produce. How are bodies thrown about, and even heavy stones cast to a great distance, by the stroke! I do not deny but that the air may also have some share in this effect; but I believe it is chiefly that of concentrating the fire and keeping it together.

6. There are experiments to shew that fire, as an actual substantial fluid, is transfused with different circumstances, and some very unexpected ones, from one parcel of matter to another. If hot water, in any measure, is added to the same measure of quicksilver which is cold, the water will give about twice as much heat to the quicksilver, as the quicksilver with the same heat would give to cold water*. How can this be, unless fire is an actual fluid, which has more

room

This is described more particularly in the Essay on the First Principles of Natural Philosophy, p. 139-142.—Experiments of the same kind with these which I made above twenty years ago, have been repeated and diversified, to prove the transfusion of fire, by Dr. Crawford, in a very ingenious pamphlet upon the nature of animal heat.