PROFESSOR POWELL ON THE REPULSIVE POWER OF HEAT. 487 



stantly descended in the scale, and soon vanished. These tints, then, may be em- 

 ployed to furnish an exact indication of the most minute changes of distance between 

 the surfaces, by whatever cause they may be produced ; and the effect due to cur- 

 vature by heat (or rather, in this case, the restoration of the bent glass to a plane 

 figure,) might be calculated, and compared with the effect observed. I made many 

 experiments in this way, and satisfied myself that the change of figure was insufficient 

 to account for the whole observed effect, and that the separation indicated by the 

 descent of the tints in the scale was therefore, in part, due to a real repulsion. 



But I do not detail these experiments, because it is immediately evident that the 

 use of lenses would afford a simple mode of deciding the question, divested of all in- 

 fluence of change of figure, without any calculation. It is evident, that if the rings 

 be formed between a convex surface and one which is either convex, plane, or even 

 concave of less curvature, heat applied outside of either glass will tend, by the change 

 of figure in every case in the first instance, to diminish the angle of contact ; that is, 

 (if no other cause interfere,) to make the rings enlarge, without altering the central 

 tint, until the curvature become equal to that of the convex surface. 



In this form of the experiment I have invariably found that,y/'om the first moment, 

 the rings regularly contract, and the central tint descends in the scale, till the whole 

 vanishes. 



There are, however, several precautions necessary to be attended to. If the glasses 

 be more than very slightly convex, the portion of the surface, throughout which they 

 approach sufficiently close for the repulsion to act, is very small : this may render 

 the total effect of the repulsive force too weak to overcome the weight of the upper 

 glass, or even its inertia, though placed vertically. This difficulty I found with sur- 

 faces which gave the first bright ring, when the centre was a point of maximum 

 brightness, about 0*1 inch diameter. Even here the rings never enlarged. But with 

 surfaces of less curvature, which gave a diameter of 0*2 or 0*3 inch, the effect never 

 failed to be exhibited, most decidedly, on bringing a red hot iron over the glasses 

 when laid one on the other, without pressure. 



The experiments, though simple in principle, certainly require some care: but with 

 all precautions, and after the most careful consideration of all causes which can have 

 tended to produce or affect the result, it appears to me that the separation of the 

 glasses through the extremely small but finite and known spaces, whose changes are 

 indicated by the degradation of the tints, can only be due to the real action of a re- 

 pulsive power, produced or excited between the surfaces of the glasses hy the action of 

 heat. 



There are many questions relating to the nature and properties of this repulsive 

 power which are immediately suggested, and some of which appear capable of so- 

 lution by variations of the same method. 



The distance at which the repulsive power can act is shown by these experiments 

 to extend beyond that at which the most extreme visible order of Newton's tints is 



3 R 2 



