282 Mr KELLAND, ON THE MOTION OF 



30. I am not aware that the transmissibility of these substances 

 has been accurately ascertained : it appears, as far as I liave had oppor- 

 tunity of judging, that they exactly coincide with the above table. 



But further, the velocity of transmission of the heat depends on 

 the quantity h\ according to the view of the subject which I have 

 given above. We ought then to find the refractive index for heat, 

 or rather that for the point of greatest heat in the spectrum, dimi- 

 nishing as Z»- increases, that is, as I in my former paper diminishes : 

 the above table then must represent the order in which the points of 

 greatest heat deviate from direct transmission, beginning with that of 

 least deviation. 



This subject has not been examined so extensively as to enable me 

 to compare the results of theory with those of observation, numerically. 



M. Seebeck's results for Water, Crown Glass and Flint Glass, 

 coincide with the above, and they are the only ones which he has 

 given for Fraunhofer's substances. 



51. It would be leading me too far into loose speculation, were 

 I to proceed to consider the effect produced in the refractive index 

 by the increase of temperature. In fact, we have so few experiments, 

 by which such speculations could be guided, that it would be almost 

 impossible to enter upon this subject. If we had a variety of sub- 

 stances, whose specific heat was well determined, and refractive indices 

 known very accurately, it might be possible to trace the analogy that 

 exists between light and heat with considerable accuracy. Admitting 

 that the two fluids (which we have, for the sake of distinction, desig- 

 nated ether and caloric) are what we usually mean by those terms, 

 it appears from the investigation that a traniifer of caloric corresponds 

 to an expulsion of ether. Hence, if the temperature (/. e. the density 

 of the surrounding ether), and also the density of the body remain 

 the same, whilst the latent heat is increased, we should expect the 

 ether proportionably diminished, and anticipate a corresponding increase 

 of the refractive index. 



Of course I am guided, in saying mcrease of the refractive index, 

 by the hypothesis that the refraction increases as the density of the 



