been extended, seem to have been very fairly and properly conducted. We shall therefore assume the facts stated as the result of these experiments; that is, the projecting powers of the blacking, of the paper surface, of the glass surface, and of the clear metallic tin surface, as the foundation of our subsequent observations.

We shall now pass on to the 5th Experiment, which was obviously made for the purpose of confirming the preconceived theory, mentioned in the preface to his book, viz. "That the com"munication of heat among insulated bodies is " performed only by the medium of the inter"vening air."

In the beginning of the 3d chapter, by way of preface to the 5th Experiment, Mr. Leslie says,

"Provide a light frame of wood, wider than "the diameter of the largest reflector, that is, "about sixteen inches square, with feet to make "it stand perpendicular. The purpose of it is "merely to serve as a screen, having, as occa"sion requires, different thin substances attached "to it.'

EXPERIMENT V.

"Over this frame extend a sheet of tinfoil, " and having arranged the apparatus as before,

"the canister presenting its blackened surface, "set the screen parallel to it, and advanced about

two inches from it. The effect upon the fo❝cal ball will now be completely intercepted; at least, if any impression be made at all, it is too minute to be discerned.'

There is certainly nothing appears in this experiment to support Mr. Leslie's speculative opinion, that the rays of heat proceeding from a beated body are different from those of light; because it is a well-known fact, that a sheet of tinfoil does obstruct the rays of light, as well as those of heat.

EXPERIMENT VI.

"Things being disposed as before, remove the "tinfoil from the screen, and substitute a pane "of crown-glass. A very material change will "be now perceived. The liquor of the differ"ential thermometer will rise to 20 degrees, "estimating the entire and unobstructed effect "of the blackened side at 100."

The result of this experiment does not still afford any proof that Mr. Leslie's speculative opinion is well founded; but on the contrary, it affords strong proof that the rays of heat, are of the same nature as those of light. For Mr.

of

Leslie here states it as a positive fact, being the result of this experiment, that one fifth part the rays of heat proceeding from the blackened side of the canister was actually transmitted to the reflector through the glass. He also acknowledges that light in its passage through glass suffers

a similar degree of diminution. He says, Though light permeates glass and other dia"phonous substances, it yet suffers in its passage a certain degree of diminution or absorp❝tion."

In this experiment, therefore, Mr. Leslie furnishes us with very strong evidence against himself, which it is necessary we should keep`in mind.

EXPERIMENT VII.

"The apparatus still remaining in the same "situation, carry the pane of glass successively "forwards, keeping it constantly parallel and " opposite to the blackened side of the canister. "At each remove, the impression upon the focal "ball will regularly diminish; insomuch that, "when the screen has gained a position, one "foot advanced from the canister, and consequently two feet from the reflector, it will not "exceed the thirtieth part of the full effect." In this experiment he shows us, and positively

declares, that although the effect of the blackened side of the canister upon the focal ball of the thermometer be diminished, by removing the pane of glass to the distance of one foot from the canister, a certain portion of the rays of heat proceeding from that side of the canister, did still pass through the glass to the reflector, sufficient to produce an effect upon the focal ball, equal to one-thirtieth part of the full effect. This diminution in the effect, occasioned by removing the pane of glass to a greater distance from the reflector, can be very easily and clearly accounted for. But for this purpose it will be 'necessary to recall the attention of the reader to the first Experiment. In that Experiment, Mr. Lesslie describes the different sides of the canister which he then employed, to be coated with different substances, viz. one side was covered with a coat of black paint, another with a sheet of writing-paper, a third with a plate of glass, and, the fourth side remained in its natural state, having a clear metallic surface. And it appeared from that experiment, that these different surfaces possessed very different powers in the projection of the rays of heat which proceeded from the heated body contained in the canister. Mr. Leslie says, that the black surface raised the co

loured liquor in the thermometer to 100 degrees, the paper surface to 98 degrees, the glass surface to 90 degrees, and the clear metallic surface to only 12 degrees. It is, therefore, evident from this experiment, that each of the sides of this square canister, covered with these different:substances, must exert their respective powers within certain portions of that circle in the centre of which the canister is placed. To discover the bounds to which the powers of these different sides of the canister extend, let us draw the lines e, f, r, q, (see plate 1.) from the centre of the canister G, and through the four corners of it, to the circumference of the circle EHIK. These lines obviously divide the circle into four equal different parts: and it is as obvious, that the four sides of the canister covered with these different substances, will produce their respective effects mentioned above, throughout the whole of that quarter of the circle in which they are placed. And as ino straight line drawn from the centre of the canister through any of these different surfaces or different sides of it, can extend beyond the limits of these bounding lines, it is obviously impossible for any of these different sides of the