periment, which deserves attention. We know that a greater quantity of light is transmitted through glass than paper; but we find from this experiment, that paper transmits more of of heat, proceeding from a heated body, rays the The cause of this is certainly not obvious; but no conclusion can be drawn from it in favour of Mr. Leslie's speculative opinion. Glass, we know, is a much more powerful reflector than paper, and if paper reflect less, it will of conseabsorb more; and probably transmit more and although a great part of those rays, which have passed through the paper, are not visible to our senses as light, yet they may still perhaps be found to exist, by the effect produced upon the thermometer. quence, In the fifth Experiment it is demonstrated, that the rays of heat proceeding from a body heated to the degree of boiling water, are not transmitted through a sheet of tin-foil, when detached from the canister in which the water is contained, in such quantities as to produce any sensible effect upon the thermometer. But it is declared in the 6th, 7th, and 8th Experiments, that those rays of heat were transmitted to the reflector, through a screen composed of glass or paper. Being, however, determined to support his own preconceived opinion; and being seemingly altogether insensible of these truths which he had just before uttered, Mr. Leslie proceeds to draw a general conclusion from these experiments, He says, "What then is this calorific and frigorific "fluid after which we are inquiring? It is in"capable of permeating solid substances. It "cannot pass through tin, nor glass, nor paper. It is not light, it has no relation to 66 æther, it bears no. analogy to fluids, real or imaginary, of magnetism and electricity. But "why have recourse to invisible agents ?" "Quod petis hic est. "It is merely the ambient air." Here we have Leslie, contra Leslie. Which of them are we to believe? The first, like á faithful and true witness, simply declares the truth that was conveyed to his mind by the organs of sense. He says, That, in the 6th Experiment, one fifth-part of the rays of heat proceeding from the canister, were actually transmitted through a pane of glass to the reflector. And in the 8th Experiment, That one fourth part of these rays were actually transmitted through a sheet of writing-paper. But the other Leslie, being determined to support his own speculative opinion, and in direct contradiction to the testimony of his own senses, says, These rays of heat cannot pass through paper, or through glass. In one part of the passage before quoted, we are told, that the rays of heat proceeding from a heated body, bear no analogy to fluids real or imaginary; and yet in the very next sentence it is as positively affirmed, that it is merely the ambient air. Is not the air in which we breathe a fluid? This book of Mr. Leslie's, and particularly the experimental part of it, is such a composition of contradictions; such a mixture of truth and speculative opinions, as would induce any person who reads it over with attention, to believe that it must have been composed by two different persons of the same name, but of very different characters; that the one has undertaken the drudgery of performing the experiments, and fairly stated the truth, as it appeared to his own senses; and that the second Leslie, having ' set out with a speculative opinion of his own, : is constantly endeavouring to support that opinion by perverting those truths which the other had stated. For the purpose of marking more distinctly the parts which these two opposite characters have acted in the composition of this book, we shall call the one Mr. Truth, and the other Doctor Speculative. Mr. Truth now goes on with eleven other Experiments, and proves by the 10th, 11th, 12th, and 13th Experiments, that the rays of heat proceeding from a body heated to the degree of boiling water, do actually penetrate through plates of tin, when these plates are covered with another substance, whose reflecting power is less than that of tin: and that they will even penetrate through a deal-board of an inch in thickness, in such quantities as to produce an effect upon the thermometer equal to 15 degrees. Dr. Speculative, however, still endeavours, by a particular mode of reasoning, drawn from his, speculative mathematics, to pervert those truths which the other has stated in these experiments, upon the testimony of his own senses; and boldly affirms, that these experiments are positive proofs of the truth of his own speculative opinion, that the rays of heat, E proceeding from a heated body, cannot pass through any solid body. Our good friend Truth has already shown us the cause why the rays of heat were transmitted through the tin-foil, and through the plates of tin as stated in the 10th and 11th Experiments. He has shown us, that the power which any body possesses of transmitting heat, is always inversly in proportion to its power of reflection; and that those bodies which reflect most, uniformly transmit least. He showed us in the 4th Experiment, that the reflecting power of tinfoil is ten times greater than that of glass. He showed us also in that Experiment, that the reflecting power of any particular body is vested in its anterior surface; and that the reflecting power of the glass mirror, remained the same, after the silvering was rubbed off from the back of it. In the 6th Experiment he showed us, that four-fifths of the rays of heat proceeding from the black side of the canister were reflected from the surface of the glass, and that only one-fifth part was transmitted through it to the reflector. Now, as it appears from the 4th Experiment, that the reflecting power of the silver coating was entirely destroyed by the intervention of |