488 PROFESSOR POWELL ON THE REPULSIVE POWER OF HEAT. 



formed. But I have also repeated the experiment successfully with the colours 

 formed under the base of a prism placed upon a lens of very small convexity ; and 

 according- to the analysis of these colours given by Sir John Herschel='^j the distance 

 is here about the llOOdth of an inch. 



Beyond these small distances other methods must be resorted to. But the cer- 

 tainty of the result within these limits, confirms its probability at greater distances, 

 as inferred by Fresnel and Saigey. 



I have tried many experiments with the view of ascertaining the relations of the 

 repulsion by heat to different substances and conditions of surface. There are obvious 

 difficulties in the way of such experiments, except in a few cases. I have found that 

 the effect is produced not only between two surfaces of glass, but between glass and 

 metal. I applied heat at the back of a plate of speculum metal, with a highly polished 

 surface, on which the rings were formed with a convex lens ; and on comparing the 

 effect in this case with that similarly produced when a plate of glass of the same 

 thickness was employed, the effect was decidedly less with the metal, notwithstanding 

 its better conducting power : but its highly polished surface rendered it a much worse 

 radiator. 



In attempting to repeat the experiment with a coated or roughened surface, there 

 is the radical difficulty of rendering the rings visible. There are obvious objections 

 to coating the polished surface, and leaving a small space clear in the centre to form 

 the rings, owing to inequality of surface and contact. But I have found that this is 

 not absolutely necessary. The rings' may be formed if the central part of the coating 

 be only slightly rubbed, and particles of the coating left adhering. I have formed 

 the rings when such particles are seen in the middle of them. With this precaution 

 I tried many comparative experiments. The metal plate coated with China ink gave 

 a greater effect than when plain. With a plate of glass coated with China ink, the 

 smoke of a candle, and leaf-gold, the effect with the smoke and China ink was greater 

 than with the leaf-gold, which accords with the greater radiating power of those 

 surfaces. 



But with all these coatings the effect was greater than with the plain glass. 

 Whereas, according to Sir John Leslie, both the China ink and the leaf-gold have 

 lower radiating powers than the glass. This difference I ascribe to the better con- 

 tact which the lens has with the softer and more yielding surface of the coating 

 against which it is pressed. 



These comparative experiments were made by placing the plate, with the lens rest- 

 ing on it, at an invariable height above the flame of a spirit lamp. 



From these experiments, then, though we may infer that, cceteris paribus, the better 

 radiating power of the surface increases the effect, yet there are other circumstances 

 which affect the result more powerfully, and these seem to be, in general, whatever 

 may tend to the more rapid communication of heat. 



* On Light, p. 641. 



