10 
BRITISH ASSOCIATION. —1835. 
ter was heated 10 or 12 degrees beyond the temperature of the 
room. The effects were now found to vary according to the di¬ 
stance of the canister from the mirror. At a short distance it acted 
as a cold body, and the varnished side most efficient; on increasing 
the distance, the effect diminished, and at a certain point altogether 
ceased; the thermometer marking zero, whether the varnished or 
metallic side was towards it; but on increasing the distance, the 
canister began to act as a warm body, and again the varnished side 
displayed its superiority. Gtli, 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 phenomena, 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. 
Dr. Hudson thenmentioned a few experiments made with Melloni’s 
thermo-multiplier, respecting the question of the “ direct free trans¬ 
mission 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 
