RADIANT HEAT. 345 



transmit most heat of high temperature or high refrangibility, analo- 

 gous to violet light: 



1. Several pure metallic powders. 2. Rock salt, in powder, and 

 many other powders, 3. Animal membrane. 



IV. Heat of low temperature is most regularly reflected at imper- 

 fectly polished surfaces. It is also, as has been shown above, most 

 regularly transmitted. , These facts are in themselves very remarkable, 

 and especially so with reference to the theory of heat, and its analogies 

 to that of light, particularly with respect to absorption. Some of 

 these considerations, which bear on the modulatory doctrine, are 

 noticed by the author in section 24. 



The curious question relative to the analogies of the action of 

 gratings, &c, to the parallel cases in the interference of light, has 

 been recently illustrated by some mathematical investigations by 

 Professor Kelland; and the author concludes his memoir with some 

 highly ingenious and interesting suggestions for further inquiry bear- 

 ing on these topics. 



Radiation of Heat : Hudson. 



At the meeting of the British Association, 1835, Dr. Hudson, of 

 Dublin, communicated some researches on radiant heat, of which 

 notices appear in the report of that meeting. — (p. 163, and Proceed- 

 ings of Sections, p. 9.) A paper by the same author on the subject 

 is printed also in the London and Edinburgh Journal of Science, vol. 

 viii, p. 109. 



In the paper last mentioned, besides making some critical remarks 

 on the results of Melloni and others, the author describes a very simple 

 and effective mode of arranging the apparatus for experiments on dia- 

 thermancy with the thermo-multiplier, so as completely to exclude the 

 influence of secondary radiation. The source of heat is a canister of 

 hot water, which can be so placed in two different positions that it is 

 exactly at the same distance, and presents the same surface; but in 

 one case the pile receives the heat both direct and secondary; in the 

 other only the secondary, derived from the heating of the screen. 



In his communication to the British Association the same author 

 examines principally certain questions bearing on the supposed radia- 

 tion of cold, and the theory of Leslie. These were performed by a 

 differential thermometer, and a concave reflector, with a hollow back, 

 so that the mirror itself could be heated to any required point by 

 filling the hollow with hot water. The source of heat was a canister 

 of water, with one surface varnished, another metallic. 



The main results were as follows: 



1. The mirror being at the temperature of the air, and the canister 

 cooled btloiv it, the varnished side produced a greater cooling effect on 

 the focal bulb than the plain, in the same ratio as that in which it 

 produced a greater heating effect when the canister was heated above 

 the air. 



2. The mirror being heated to 200° Fahrenheit, and the canister 

 at the temperature of the air, both bulbs were so placed as to be 



