NATURAL PHILOSOPHY. 



tium, the colors diminish in rcfrangibility as the temperature rises; but the 

 reverse is the case with some other substances. The different effects depend 

 upon the particular molecular condition of each substance. Cumpks Rcn- 

 dus, xlvii. 104. 



EXPERIMENTS OX RADIANT HEAT. 



The following account of some experiments on radiant heat, involving: an 

 extension of Prevost's theory of exchanges, was presented to the British As- 

 sociation, at the last meeting, by Mr. B. Stewart. The experiments in ques- 

 tion were performed with the aid of the thermomultiplicr, the source of heat 

 being for the most part bodies heated to 212. Four groups of experiments 

 were considered. Group the first contains those experiments in which the 

 quantities of heat radiated from polished plates of different substances at 

 a given temperature, are compared with the quantity radiated from a similar 

 surface of lampblack at the same temperature. The result of this group of 

 experiments is, that glass, alum, and selenite radiate about 08 per cent, of 

 what lampblack does; thick mica, 92; thin mica, 81; and rock-salt only 

 15 per cent. The second group of experiments was designed to compare 

 together the quantities of heat radiated at the same temperature from pol- 

 ished plates of the same substance, but of different thicknesses. The result 

 of this group was, that, while the difference between the radiating power of 

 thick and thin glass is so small as not to be capable of being directly 

 observed, there is a perceptible difference between the radiation from thick 

 and thin mica, and a still more marked difference between the radiation 

 from plates of rock-salt of unequal thickness. The third group of experi- 

 ments was made with the view of comparing the radiations from various 

 polished plates with that from lampblack, as regards the quality of the heat, 

 its quality being tested by its capability of transmission through a screen 

 of the same material as the radiating plate. From this group of experiments 

 it appears that heat emitted by glass, mica, or rock-salt, is less transmissible 

 through a screen of the same material as the heated plate than heat from 

 lampblack, this difference being very marked in the case of rock-salt, 

 which only transmits about one-third of the rays from heated rock-salt. 

 The common opinion that rock-salt is equally diathermanous for all descrip- 

 tions of heat, is therefore untenable. The fourth group of experiments 

 shows that heat from thick plates of glass, mica, or rock-salt, is more easily 

 transmitted by screens of the same nature as the heated plate than heat 

 from thin plates of these materials. It was shown that all these experi- 

 ments may be explained by Prevost's theory of exchanges, somewhat ex- 

 tended. This extension consists of the following laws: 1. Each particle 

 of a substance has an independent radiation of its OAVU, equal in all directions, 

 and without regard to the distance of the particle from the surface of the 

 body. 2. The radiation of a particle equals its absorption, and that for 

 every description of heat. 3. The flow of heat from within upon the interior 

 surface of a polished plate of indefinite thickness, is proportional to the index 

 of refraction of the body, and that for every description of heat. The bear- 

 ing of these experiments on Dulong and Petit's law of radiation was then 

 attempted to be traced. It was shown that unless bodies from simply being 

 heated change their transmissibility for the same description of heat (which 

 there is no reason to suppose), the radiation of thin plates or particles at a 



