254 Professor Powell on the Repulsion prQduced [April 



towards the exterior parts, which tends to prevent them 

 from being depressed. When the curvature begins to change, 

 therefore, there is somewhere between them a neutral or 

 nodal point whose position does not change ; this point may 

 be very near, or even in the centre, when the attraction is 

 very strong there. A remarkable instance of this occurs 

 when the first black of the scale is formed between glass 

 plates, and heat carefully applied exactly over the central 

 point of the black space ; in this case, when the black space 

 is a | inch or more in diameter, I have often continued the 

 application of the strongest heat for a great length of time 

 before any separation could be effected, when at length it 

 has taken place with a sudden force and an audible click. 

 Sometimes the black spot has continued unaltered until the 

 glasses have cracked, when the fragments have still con- 

 tinued to adhere powerfully : meanwhile the outer rings 

 have continued gradually enlarging. 



In the foregoing statement, I have observed, that in 

 using plane glasses, it was necessary to allow for the effect 

 of warping. But there are certain considerations which 

 show that that precaution is unnecessary. For, acccording 

 to the beautiful experiments of Sir D. Brewster on the 

 progress of heat through glass, as evinced by its action on 

 polarized light, it appears distinctly, that the change of 

 structure (if I may so speak) in the molecules of the glass is 

 produced at the same instant, on both sides of the plate : so 

 that the effect of warping cannot take place. This is ren- 

 dered evident to the eye, by the symmetrical arrangement 

 of the luminous bands, from the first moment of the appli- 

 cation of heat, on each side of the dark central band, which 

 occupies the neutral line along the middle of the thickness 

 of the glass. 



When two plates of glass are laid upon one another there 

 is a certain resistance or repulsion which may be overcome 

 by pressure. We can press them together till attraction 

 takes place. On removing the pressure they remain adher- 

 ing. If we press them more they are brought closer, and 

 produce the colours of thin plates. We may thus produce 

 successively any given tint, and on removing the pressure 

 that tint will remain, or the glasses continue in the same 

 position to which they have been brought. 



This seems to show that the attraction and repulsion are 

 in exact equilibrio at all distances, (within this range,) and 



