206 HEAT SECT. XXV. 



SECTION XXV. 



Heat Calorific Rays of the Solar Spectrum Experiments of MM. De 

 Laroche and Melloni on the Transmission of Heat The Point of greatest 

 Heat in the Solar Spectrum varies with the Substance of the Prism 

 Polarization of Heat Circular Polarization of Heat Transmission of the 

 Chemical Rays Absorption of Heat Radiation of Heat Dew Hoar 

 Frost Rain Hail Combustion Dilatation of Bodies by Heat Propa- 

 gation of Heat Latent Heat Heat presumed to consist of the Undula- 

 tions of an Elastic Medium Parathermic Rays Moser's Discoveries. 



IT is not by vision alone that a knowledge of the sun's 

 rays is acquired, touch proves that they have the 

 power of raising the temperature of substances exposed 

 to their action. Sir William Herschel discovered that 

 rays of caloric which produce the sensation of heat, exist 

 in the solar spectrum independently of those of light ; 

 when he used a prism of flint-glass, he found the warm 

 rays most abundant in the dark space a little beyond the 

 red extremity of the spectrum that from thence they 

 decrease toward the violet, beyond which they are in- 

 sensible. It may therefore be concluded, that the ca- 

 lorific rays vary in refrangibility, and that those beyond 

 the extreme red are less refrangible than any rays of 

 light. Since Sir William Herschel's time it has been 

 discovered that the calorific spectrum exceeds the lumi- 

 nous one in length in the ratio of 42 to 25, but the most 

 singular phenomenon of the calorific spectrum is its 

 want of continuity. Sir John Herschel blackened the 

 under side of a sheet of very thin white paper by the 

 smoke of a lamp, and having exposed the white side to 

 the solar spectrum, he drew a brush dipped in spirit of 

 whie over it, by which the paper assumed a black hue 

 when sufficiently saturated. The heat in the spectrum 

 evaporated tha spirit first on those parts of the paper 

 where it fell with greatest intensity, thereby restoring 

 their white color, and thus he discovered that the ca- 

 loric is not distributed uniformly, but in spots of greater 

 or less intensity a circumstance probably owing to the 

 absorbing action of the atmospheres of the sun and 

 earth. " The effect of the former," says Sir John, u is 

 beyond our control, unless we could carry our experi- 

 ments to such a point of delicacy as to operate separately 



