2G4 THERMOMULTIPLIER. SECT. XXVII. 



comes to the anterior face of the prism of water consists of rays 

 of all degrees of refrangibility. Now, the rays possessing the 

 same index of refraction with the red light suffer a greater loss 

 in passing through the prism than the rays possessing the 

 refrangibility of the orange light, and the latter lose less in their 

 passage than the heat of the yellow. Thus the losses, being 

 inversely proportional to the degree of refrangibility of each ray, 

 cause the point of maximum heat to tend from the red towards 

 the violet, and therefore it rests upon the yellow part. The 

 prism of sulphuric acid, acting similarly, but with less energy 

 than that of water, throws the point of greatest heat on the 

 orange ; for the same reason, the crown and flint glass prisms 

 transfer that point respectively to the red and to its limit. M. 

 Melloni, observing that the maximum point of heat is transferred 

 farther and farther towards the red end of the spectrum, accord- 

 ing as the substance of the prism is more and more permeable to 

 heat, inferred that a prism of rock-salt, which possesses a greater 

 power of transmitting the calorific rays than any known body, 

 ought to throw the point of greatest heat to a considerable dis- 

 tance beyond the visible part of the spectrum, an anticipation 

 which experiment fully confirmed, by placing it as much beyond 

 the dark limits of the red rays as the red part is distant from the 

 blueish green band of the spectrum. 



In all these experiments M. Melloni employed a thermomulti- 

 plier, an instrument that measures the intensity of the trans- 

 mitted heat with an accuracy far beyond what any thermometer 

 ever attained. It is a very elegant application of M. Seebeck's 

 discovery of thermo-electricity ; but the description of this 

 instrument is reserved for a future occasion, because the principle 

 on which it is constructed has not yet been explained. 



In the beginning of the present century, not long after M. 

 Mains had discovered the polarization of light, he and M. Berard 

 proved that the heat which accompanies the sun's light is capable 

 of being polarized ; but their attempts totally failed with heat 

 derived from terrestrial, and especially from non-luminous 

 sources. M. Berard, indeed, imagined that he had succeeded ; 

 but, when his experiments were repeated by Mr. Lloyd and 

 Professor Powell, no satisfactory result could be obtained. M. 

 Melloni resumed the subject, and endeavoured to effect the 

 polarization of heat by tourmaline, as in the case of light. It was 



