314 Proceedings of the British Association. 



even from non-luminous sources. He found its transmissive 

 power six or eight times greater than that of an equal thickness 

 of alum, which had nearly the same transparency and refractive 

 power ; and that, unlike other diathermanous media, it is equally 

 diathermanous to every species of heat, i. e. whether from sources 

 highly heated or moderately heated ; thus, he found a plate of 

 7 millimetres (.28 inch) thick, to transmit 92 out of 100 rays, 

 whether from flame, red hot iron, water at 212°, or at 120'^ F. 

 A plate one inch thick gave a similar constant ratio : the general 

 conclusion resulting, that the source being a lamp, the diather- 

 mancy is not proportional to the transparency; and he makes 

 some general remarks on these results, as related to those of See- 

 beck, on prismatic dispersion. In a supplementary paper, Melloni 

 investigates the modifications which calorific transmission un- 

 dergoes in consequence of the radiating source being changed. 

 He employs four sources of heat : — 1, a Locatelli lamp; 2, incan- 

 descent platinum ; 3, copper heated by flame to about 730° F. ; 

 4, hot water in a blackened copper vessel. The discovery of the 

 complete diathermancy of rock-salt furnished the means of pros- 

 ecuting the author's researches on the refraction of heat. In the 

 successful experiment which he made, he concentrated in the fo- 

 cus of a rock-salt lens, the rays of dark heat from hot copper and 

 hot water. A similar lens of alum produced no effect, which 

 proves that the efl'ect is not due to the mere heating of the cen- 

 tral part of the lens. In discussing the properties of the calorific 

 rays transmitted by different bodies, a remarkable effect presented 

 itself; the rays of the lamp were thrown upon screens of differ- 

 ent substances in such a manner that, either by changing the dis- 

 tances, or by concentration with a mirror or a lens of rock-salt, 

 the effect transmitted from all the sources was of a certain con- 

 stant amount. This constant radiation was then intercepted by 

 a plate of alum, and it was found that very different proportions 

 of heat were transmitted by the alum in the different cases ; from 

 which Melloni concludes, " that the calorific rays issuing from 

 the diaphanous screens are of different qualities, and possess (if we 

 may use the term,) the diathermancy peculiar to each of the sub- 

 stances through which they have passed." He next investigated 

 the effects of colored glasses, and concludes that all the colored 

 glasses except green, produce no 'elective action' on heat ; green 

 glass, on the contrary, transmits rays more easily stopped than 



