298 British Association for the Advancement of Science. 



canister began to act as a warm body, and again the varnished side 

 displayed its superiority. 6th, When the focal ball (merely) was 

 cooled by the evaporation of water, or even of aether, neither side of 

 the canister produced any change in the effect. 7th, When the 

 focal ball was cooled 27° of Fahrenheit (by evaporation of aether), 

 and the canister cooled 16° of Fahrenheit (being of course 11° 

 warmer than the focal ball), the focal ball was now cooled more 

 than previously, as if the canister were (comparatively) a cold body. 

 The rapid evaporation of the aether makes these experiments trouble- 

 some. The author then pointed out that no theory of the emission of 

 rays of heat could account for the phaenomena, unless rays of cold 

 were also admitted ; and called attention to Professor Leslie's theory, 

 as deserving further investigation, without, however, drawing any 

 conclusion from the experiments, excepting that they could only be 

 accounted for on some theory of undulations. He then suggested, 

 as one cause of the different radiating powers of surfaces, their dif- 

 ferent capacities for heat. The two surfaces being at the same tem- 

 perature and in the same medium (of a lower temperature), may be 

 considered to have the same tendency to attain the common tempe- 

 rature of the medium, and may therefore be expected to give off the 

 same portion of their excess of temperature, and consequently quan- 

 tities of heat proportional to the capacities of the surfaces ; taking 

 the latter in the physical sense of having some definite thickness, 

 which may be different in different substances. 



He then mentioned a few experiments made with Melloni's ther- 

 mo-multiplier, respecting the question of the " direct free transmis- 

 sion of heat " through rock-salt, rock-crystal, and alum. Having 

 removed the crystals from the opening in the screen, he moved the 

 canister (containing hot water) entirely out of the axis of the ther- 

 moscope, so that the needle stood at zero. He then placed the 

 crystals (successively) in the opening of the screen, and found the 

 effects on the needle to be instantaneous, and also to follow the 

 same order in the different crystals as to its amount, as when the 

 canister was in the axis of the thermoscope, so as to make it question- 

 able whether the effects in the latter case were not (also) wholly 

 owing to the conduction of heat through the crystals. He alluded 

 to these experiments merely as indicating a method of determining 

 the point in question : as, if there be (contrary to Melloni's deduc- 

 tions) no direct transmission of simple heat, we may expect to find 

 the same results produced by a given source of heat, whether in or 

 out of the axis of the instrument, provided the canister and the cry- 

 stal are equally distant, and their surfaces equally inclined to each 

 other in both cases. In the experiments with the mirror, he had 

 used a differential thermometer containing aether instead of sulphuric 

 acid, as being much more delicate in its indications of heat ; and 

 suggested its being made still more sensitive by the use of other 

 liquids, having himself succeeded in making one containing con- 

 densed sulphurous acid gas. 



