COLOURS OF THIN PLATES, 129 



colours begin to be exhibited at the top, where it is thinnest. 

 These colours arrange themselves in horizontal bands, and 

 become more and more brilliant as the thickness diminishes ; 

 until finally, when that thickness is reduced to a certain 

 limit, the upper part of the film becomes completely black. 

 When the bubble lias arrived at this stage of tenuity, cohesion 

 is no longer able to resist the other forces which are acting 

 on its particles, and it bursts. 



Similar phenomena may be observed when a drop of oil is 

 let fall on water. As the oil spreads rapidly over the surface, 

 it is soon reduced to a very thin film, which displays the 

 spectral colours. 



Every one has noticed the fact that steel and other metals, 

 when polished, acquire various shades of colour by exposure 

 to the air. These colours are produced by a thin coating of 

 metallic oxide, which is gradually formed on the surface. The 

 formation of this oxide is greatly accelerated by an augmenta- 

 tion of temperature ; and the colour thus formed is so inva- 

 riably connected with the thickness of the film, and this latter 

 with the degree of heat, that artists are in the habit of mea- 

 suring the temperature by the colour developed. Thus steel, 

 in the process of tempering, is said to have received a yellow 

 heat, a blue heat, &o. 



The same appearances are displayed in a still more strik- 

 ing manner by air itself, or even by a vacuum. If two 

 plates of glass be pressed together by the fingers, we shall 

 observe, round the point of nearest approach, a succession of 

 coloured bands of great brilliancy, which dilate as the 

 pressure is increased, and the inclosed plate reduced in thick- 

 ness. 



(142) In order to observe these phenomena, in such a 

 manner as to be enabled to trace their laws, we must follow 

 Newton. Newton's experiment consisted simply in laying a 



K 



