238 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 Propagation of Heat Latent 

 Heat Heat presumed to consist of the Undulations 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 in- 

 dependently 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 towards the violet, beyond which 

 they are insensible. It may therefore be concluded that the 

 calorific 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 luminous 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 Her- 

 schel 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 wine over it, by which the paper assumed a black hue 

 when sufficiently saturated. The heat in the spectrum eva- 



