248 MAXIMUM OF HEAT IN THE SPECTRUM. [SECT. xxv. 



spectrum, according 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 distance beyond the 

 visible part of the spectrum, an anticipation which experi- 

 ment 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 thermo- 

 multiplier, an instrument that measures the intensity of the 

 transmitted 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 occa- 

 sion, because the principle on which it is constructed has 

 not yet been explained. 



In the beginning of the present century, not long after 

 M. Malus 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 lately resumed the 

 subject, and endeavoured to effect the polarization of heat 

 by tourmaline, as in the case of light. It was already shown 

 that two slices of tourmaline, cut parallel to the axis of the 

 crystal, transmit a great portion of the incident light when 

 looked through with their axes parallel, and almost entirely 

 exclude it when they are perpendicular to one another. 

 Should radiant heat be capable of polarization, the quantity 

 transmitted by the slices of tourmaline in their former posi- 

 tion ought greatly to exceed that which passes through them 

 in the latter, yet M. Melloni found that the quantity of heat 

 was the same in both cases : whence he inferred that heat 



