1835.] between Bodies by the Action of Heat. 253 



to the analysis of these colours given by Sir J. Herschel, (on 

 Light, 641,) the distance is here about the 1100th of an inch. 



Beyond these very small distances other methods must 

 be resorted to. But the certainty of the result within these 

 limits perhaps confirms its probability at greater distances, 

 as inferred by Fresnel and Saigey. 



I tried several experiments on the effects of different 

 sorts of surfaces, from which I conceive, though we may 

 infer that cceteris paribus, the better radiating power of the 

 surface increases the effect ; yet there are other circum- 

 stances which affect the result more powerfully, and these 

 seem to be, in general, whatever may tend to the more rapid 

 communication of heat. 



This is still more conspicuous when the rings are formed 

 in a thin plate of water between the lenses. The effect is 

 here even greater than in air, and we may presume, inde- 

 pendent of radiation. 



There are several subordinate circumstances attending 

 these results which are deserving of notice. When the 

 lenses are in close contact, there is, in all cases, a consi- 

 derable attraction opposed to the repulsive power. If the 

 central black be formed, it requires a very considerable 

 intensity of heat to overcome the attraction, which at that 

 minute distance is extremely powerful. 



When the heat is removed the colours return, and the rings 

 are gradually restored to the same character as they had at 

 first. This is more remarkable when simple plates of glass are 

 employed as before described. When the heat has restored 

 the bent glass to a plane figure, on its removal the rings 

 return, and consequently, the glass is again bent without 

 any fresh pressure, though the force originally applied to 

 produce the curvature was very considerable ; this is pro- 

 bably owing, in a great measure, to atmospheric pressure. 

 In this case, however, the colours will only return up to a 

 certain point, generally not higher than the beginning of 

 the first order. 



When two glasses are pressed together there is a repulsion 

 to be overcome, evinced by the force which it is necessary 

 to apply, and in general, it is evident, that if a plate resting 

 on another be bent by pressure, as in these experiments, the 

 influence of heat in restoring it to a plane form will be 

 opposed both by the attraction at the centre, which tends 

 to prevent that part from being raised, and by the repulsion 



