room to expand itself in the interstitial vacuities of the water, but is condensed and straitened when communicated to the closer passages of the mercury? On any other supposition, the fact is contrary to the laws of mechanics; which in all cases' allow the greater momentum to the heavier matter. And hence it is certain, that fire is not the production of a motion in the solid parts of matter; because, in that case, the heavier particles of quicksilver must communicate more motion to the parts of water, than the parts of water, which are so much lighter, and have consequently less momentum, can communicate to the parts of quicksilver : whereas the effect of water on mercury is twice as great as the effect of mercury on water; and therefore it is not to be solved by any supposed action of their parts, but by the transfusion, and expansion, and condensation of an actual fluid within their pores; which fluid is the matter of fire.

7. The materiality of the rays of light is a necessary consequence of their decomposition. Whatever is immaterial must be purs and simple: but the light is a mixt substance, consisting of rays which differ in colour, and according to their colour are differently

3

differently refrangible. As light is the emanation of fire, and that which is light on the surface of a burning glass is fire at the focus of it; what is proved of light, in respect of its materiality, must be applicable to fire.

It might be sufficient to allege upon this argument, that nothing but that which is bodily can affect the bodily senses. Light may be seen, and fire may be felt: indeed the rays of the sun affect both senses at once; the sight by their splendour, and the feeling by their heat; and in the experiments of electricity, the shock of fire is so evident to the sense, that I think nothing farther need be urged to prove that fire is a sensible object, or, in other words, a corporeal substance. And therefore we shall proceed to examine some of its properties; the first and most remarkable of which is that power it has to penetrate all other bodies, and to act as well within as without them.

On the Penetrating Power of Fire.

The passage of the rays of light through the body of glass is so well known, that it need not be insisted upon; and it is almost as obvious, that the transparency of bodies.

is entirely owing to the admission or retention of the light within their pores. But the same matter, when it is not visible by its radiancy, will be propagated freely through such bodies as are impervious to every other substance. If a thermometer is closely covered with a vessel of glass, any heat applied without side will raise the thermometer to the same height as if the glass were not interposed and a coal of wood screwed up fast in a vessel of iron, will be ignited as effectually as if it were in the naked fire; which could not possibly happen unless the matter of fire has a free passage through the sides of the vessel. Why does a thermometer mark the same degree under the exhausted glass of an air-pump, as in the open air, but because the same degree of fire which is diffused through the atmosphere, is also diffused through what we call a vacuum? When we heat a ball of clay and cut it asunder, it will be as hot at the centre as at the surface. Water, when boiling over the fire, is as hot at the top, as at the bottom, which lies contiguous to the fire: and the same would probably be true, if the vessel were carried to a considerable height.

But how certain soever it may be that fire

hath

hath the power of penetrating all other bodies, it does not appear that the different degrees of fire can penetrate them with equal forces or in equal times. Heat and cold are not names of things essentially different, but only of different degrees of the same thing*, that is, of fire in motion; and therefore we call it the penetrating power of fire, whether we examine it in that state which affects us with a sense of heat, or in that other different one which affects us with a sense of cold. In the latter state it does not penetrate bodies with the same ease as in the former. If a thermometer, having the temperature of cold water, be plunged into a vessel of hot water, it will in any given time rise through more degrees than it will be found to sink through when removed from the hot and plunged again into the cold water. This is sufficient to verify the principle, and the trial is easily made.

When this subject first occurred to me, I had the opportunity of some very severe weather, at which time I immersed a vessel of water in a freezing mixture, wherein the thermometer sunk to 30 degrees below the cypher

VOL. IX.

See the Essay, p. 160, &c.

cypher of Fahrenheit. I found that 23 degrees of cold were propagated to the centre of the included vessel in the space of five minutes. But when the same vessel, with water of the same temperature as before, was plunged into other water with an excess of heat equal to the excess of cold in the freezing mixture, the heat communicated in the same space of time amounted to 64 degrees: so that the two effects were to each other nearly as 5 to 14*.

And thus it ought to be in reason: for, when air is stirring briskly in the form of wind,

*Mr. Amontons, an ingenious author of the French academy, contrived a vessel of such a structure as to shew by inspection that heat and cold were not communicated in equal times, or that a fluid will acquire heat much sooner than it will part with it.

ABCD (fig. II. plate I.) is a cubic vessel of tin, divided into two equal cavities by the false bottom. IG is a tube of tin soldered into EF, and communicating with the lower region of the vessel at I, and also with the upper by means of another exterior tube soldered to CD. PQ is a tube communicating with the upper cavity at the orifice P, and also with the reservoir R. S is a vent or stop-cock, which being opened, the water that is poured into the reservoir will descend and fill the upper cavity of the vessel, as it is expressed in the figure, after which the orifice must be securely closed..

The