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Rh as the losses suffered by the rays in successively passing through the four plates of glass; it being carefully kept in mind that these values are not referred to the initial quantity, but to the number of rays which arrive at the surface of each screen.

Thus the proposition of Delaroche is true as far as the third and the fourth screens; for in the transition from one loss to another a diminution of each loss is observable.

It will have been observed that the losses were not so great in respect to the four equal layers of the screen of a fourfold thickness; and that this should happen will be easily conceived if we consider that in the latter case there is a solution of continuity which causes a greater dispersion of the heat by reflexion. But we see that in both cases the difference between two successive losses becomes less in proportion as the distance from the surface, at which the rays entered, is greater.

Let us now proceed to consider the influence exercised on calorific transmission by the composition of the substance of the screen.

M. Prevost had concluded from the experiments described in a memoir already quoted, that water and glass ought to transmit rays of heat in different quantities; for by causing the sheet of water to fall between the lighted candle and a very delicate air-thermometer, he obtained no indication of heat being transmitted unless when he had blackened the ball of the thermometer, and even then the increase of temperature was extremely small; whilst a plate of glass substituted for the sheet of water produced effects sufficiently manifest. But it was objected to him that the difference between the action of the water and that of the glass was owing to the conductible caloric which was perceptible in the latter case only. Delaroche subsequently observed that a square of greenish glass transmitted more heat than a plate of another species of glass perfectly pure. However, as the first flake was much thinner than the second, it was insisted that the difference in the effects was owing to the difference of thickness. At length, some time after the invention of the thermomultiplier, M. Nobili and myself made some experiments on olive oil, alcohol, water, and nitric acid; whence we inferred that water opposed a greater resistance than any of the three other liquids did to the passage of rays of heat emanating from a hot iron. But these experiments are to be regarded only as mere trials, tending to show the facility with which the thermomultiplier may be employed in all sorts of inquiries relative to calorific radiation; for we did not take sufficient precautions to prevent the heat from passing by