Proceedings of the British Association. 315 



the others ; and that green glass is the only kind which possesses 

 a coloration for heat, (if we may use the terra,) the others acting 

 upon it only as more or less transparent glass of uniform tint does 

 upon light. From experiments upon the solar rays transmitted 

 by green glass, and intercepted by other media, he found they 

 passed copiously through rock-salt, but feebly through alum : 

 whence he concludes that there are among the solar rays some 

 which resemble those of terrestrial heat ; and in general, that the 

 differences observed between solar and terrestrial heat, as to their 

 properties of transmission, are therefore to be attributed merely to 

 the mixture, in different proportions, of these several species of 

 rays. Prof Forbes repeated and extended Melloni's experiments 

 on the transmission and refraction of heat. One of the most in- 

 teresting points to which he directed his attention, was the possi- 

 bility of detecting heat in the moon's beams. These, concentra- 

 ted by a polyzonal lens of 32 inches diameter, and acting on the 

 thermo-multiplier, gave no indication of any effect : so that Prof. 

 F. considers it certain that if there be any heat, it must be less 

 than the 300,000th part of a degree centigrade. In his third 

 section, he investigates the index of refraction for heat of differ- 

 ent kinds as compared with that for light in the same medium. 

 The method of observation adopted was indirect, depending upon 

 the determination of the critical angle of total internal reflection 

 in a rock-salt prism with two angles of 40°, and one of 100°. By 

 an ingenious mechanical contrivance, the sentient surface of the 

 pile was made to receive rays coming from the source of heat 

 after undergoing two refractions and one reflection, whatever was 

 the angle of incidence. The mean of the results obtained from 

 various sources of heat, variously transmitted, for the index of re- 

 fraction for rock-salt, is 1.552. The results deduced are : — 1. 

 The mean quality, or that of the more abundant proportion of 

 the heat from different sources, varies within narrow limits of re- 

 frangibility. 2. These limits are very narrow indeed where the 

 direct heat of any source is employed. 3. All interposed media, 

 (including those impermeable to light,) so far as tried, raise the 

 index of refraction. 4. All the refrangibilities are inferior to that 

 of the mean luminous rays. 5. The limits of dispersion are open 

 to further inquiry, but the dispersion in the case of sources of 

 low temperature, appears to be smaller than that from luminous 

 sources. 



