and on Thin Plates. 35 



(green was made by overlapping the blue and yellow shadows.) 

 Surely, if it were a mere privation, the light of the candle 

 could only illuminate the black shadow and make it white. 

 However, the fact is, that the black shadow is condensed 

 light, and rarefied into different colours. If we suppose a 

 number of grains of shot to represent the soap atoms, placed 

 at different distances from one another, and represented by 

 a, a% a s , a 4 , &c, &c, the light passes through those and is 

 reflected from the second surface, and gives, when variously 

 condensed, this or that colour, according to the approximation 

 of these atoms, and not the thickness of the plates*. 



Ascending and Descending Currents in a Soap Film. 



On making a soap film I placed it on an inclined plane, 

 and perceived the coloured bands to descend slowly, and in- 

 creasing in breadth, until at last the attraction of cohesion 

 lost its influence, and the atoms became free and ascended in 

 currents, particularly at the concave sides of the bottle ; as they 

 rose generally white atoms, they passed through the coloured 

 bands, until after passing through blue, red, green, &c. they 

 fell into the ranks of their own colour. In boiling water we 

 perceive currents by means of powdered resin, or other light 

 substances, evidently caused by an addition of caloric. Here 

 there is no such addition, and we must look to some other 

 cause, perhaps electricity. Sometimes these atoms take an 

 elliptical or circular motion ; this is best seen by placing the 

 bottle with the plane film on the table and surrounding it 

 with the warm hand ; as the room was at 60° and my hand 

 at 80°, I threw 20° into the film. Here there can be no 

 differences of thickness ; however, almost simultaneously, the 

 force of cohesive attraction forms the coloured bands, and 

 when the film becomes white, then these beautiful and in- 

 teresting movements take place. How far these laws may 

 act on the solar system, I leave to the contemplation of the 

 astronomer. All fluids are in perpetual motion, from the 

 broad Atlantic to the permanent soap film ; and hereafter I 

 shall be enabled, by some new experiments, to show that the 

 same laws regulate the atmosphere. 



Colours of the Clouds and. Complementary Colours. 



When the soap film was entirely black, after remaining 

 perhaps an hour on the table, I placed the bottle in a basin 

 of boiling water, and in a short time perceived the film to be 



* Count Rumford, as well as others, made many experiments on co- 

 loured shadows, but entirely overlooked these changes; at last, in despair, 

 he says, it was a deceptio ocxdi. 



D2 



