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. ; 
A, 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 effect 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 ail the colored 
glasses except green, produce no ‘elective action’ on heat é 7 
: t 
glass, on the contrary, transmits rays more easily stoppe 
