the different luminous rays, and thus throws certain quantities of heat on the very spaces occupied by the different colours of the spectrum; but that in the coloured glasses and, generally, in bodies more or less diathermanous, the absorbent force does not act in the same manner as the force of refraction, which sometimes extinguishes more heat than light and at others more light than heat.
But those who maintain the identity of the two agents will reply, that the differences observed in the calorific and luminous transmissions of the diaphanous or coloured media are produced by rays of obscure heat which mix in great quantities with the rays of light emitted by the flame.
In order to decide how far it is allowable to maintain the one or the other hypothesis, we should have data which, at present, are not within our reach. We shall resume this subject at the end of the next Memoir, and conclude the present one with an account of a very remarkable application of the numerical results contained in the foregoing tables.
It had been established by the beautiful experiments of Seebeck that the place of the maximum of temperature in the solar spectrum varies with the chemical composition of the substance of which the prism is made. This eminent philosopher observed that the highest degree of heat which, in the spectrum furnished by a prism of crown glass, was in the red, passed to the orange when the prism employed was a hollow glass one filled with sulphuric acid, and was found in the yellow when the same prism was filled with pure water[1].
I discovered some months since that the caloric rays scattered on the colours given by a common prism do not undergo the same alteration in passing through a layer of water; the loss varies inversely as the refrangibility, so that the most refrangible rays pass undiminished and the least refrangible are entirely stopped by the liquid[2].
This experiment led to a very simple explanation of the results obtained by Seebeck.
The solar heat which presents itself to the anterior face of the prism of water contains rays of every degree of refrangibility. Now the ray which has the same index of refraction as the red light, suffers in passing through the prism a loss porportionally greater than the ray which possesses the refrangibility of orange light, and less is lost by the latter in the passage than by the heat of the yellow ray. These increasing ratios in the losses of heat sustained by the less refrangible rays have an evident tendency to transfer the maximum to the violet. It may therefore be stopt at the yellow.
