and on Thin Plates. 33 



had evaporated and the entire film become of a black ap- 

 pearance, what Newton likened to a hole in his bubble-, 

 transmitting almost the entire light. I now moved the bottle 

 to and from me, and in a short time the entire film was 

 clothed with silvery white, reflecting atoms, which soon 

 formed into beautiful bands of colours by means of cohesive 

 attraction; for, as they formed nearly simultaneously, they 

 could not be produced by a descending fluid, causing relative 

 thicknesses ; but perhaps it may be said, that the film was 

 thickened by evaporation of the saponaceous fluid. To ob- 

 viate this objection, I made the following new, and I presume 

 to hope, conclusive experiment. 



Experiment 2. Having procured a cylindrical glass tube, 

 about one inch in diameter, I dipped one end to the depth of 

 three inches into a saponaceous solution and formed a film, 

 which, when the tube was held perpendicularly, glided down 

 three inches. I now corked the upper orifice, to prevent 

 further descent, and laying the tube on the table, in a short 

 time the bands were formed ; when these disappeared, and the 

 film was black, I shook the tube from side to side, and in 

 a short time the black changed to a silvery white, and then 

 formed into bands of different colours. On placing it again 

 on the table the film appeared all over black. Here there 

 could be no thickening of the plates by evaporation, as the 

 air was at the bottom of the tube. This experiment, easy of 

 manipulation, I hope may be esteemed conclusive, or, as Sir 

 Isaac Newton calls it, an " experimentum crucis" As my 

 experiments are now shown in lecture-rooms, I am anxious 

 that they should be accompanied by legitimate inferences, par- 

 ticularly as some observations made by a Dublin Professor at 

 Liverpool, prevented an explanation which I now give. 



Experiment 3. Having procured a plate of very deep blue 

 glass, four inches square, I wiped it well, and then breathed 

 on it through a narrow glass tube, forming a plate of vapour, 

 which, by evaporation, went through all the relative thick- 

 nesses measured by Newton, without any variety of colours. 

 I now breathed a second time on this plate, and drew my 

 finger across the middle so as to make furrows in the plate of 

 vapour ; immediately all the variety of colours in nature was 

 formed, like threads of variegated silk. This experiment 

 evidently shows that the atoms were relatively approximated, 

 and that it was this approximation, and not any relative thick- 

 ness of the plate of vapour, which caused the colours. 



Experiment 4. On repeating the experiments of the Abbe 

 Mazeas, ^Memoires presentes" with two pieces of plane glass, 

 he justly remarks, that friction is necessary in the formation of 

 il. Mag. S. 3. Vol. 17. No. 107. My 1840, D 



