34 Dr. Reade on the Permanent Soap Films, 



coloured rings. Now it is evident that friction cannot alter the 

 thickness of a plate of air, but must bring into view some sub- 

 stances capable of condensation ; and there is nothing mixed 

 with the air capable of such condensation, except vapour. I 

 therefore must attribute the colours to this cause. Convinced 

 that vapour was the cause, I improved on the Abbe Mazeas's 

 experiment : for, on washing one of the plates of glass with 

 soap and water, and holding it before the fire, then breathing 

 on the other, by means of a slight degree of friction I co- 

 loured the entire glass, with a large black spot in the centre 

 more than an inch in diameter. When this black spot was 

 formed, the glasses were so firmly united as to require a strong 

 force to separate them. This I attribute to the air being pressed 

 out, and then the glasses acted in the same manner as a leather 

 soaker used by school-boys, and not to any cohesive attraction 

 of the surfaces, as supposed by chemists. I coated the glasses 

 with a plate of water, and on pressing them together no co- 

 lours appeared, until by a strong degree of friction I produced 

 vapour. I also smeared the glasses with some candle-grease, 

 and found no colours, until I held the glasses to the fire, 

 when, on using friction, vapour and colours were produced. 



Atomic Theory of Colours. 



Having endeavoured to prove, I hope with effect, that 

 colours are not produced by relative thicknesses of the plate, 

 as advanced by Sir Isaac Newton in the second book of his 

 Optics, I shall proceed to give what I conceive is the true 

 explanation of this interesting phaenomenon. Grimaldi and 

 others maintained that light was capable of condensation and 

 rarefaction. However, as they brought forward no experi- 

 ments to prove it, I think it unfair to say that I took my 

 theory from that celebrated philosopher. If we hold a candle 

 before a black shade made with a pencil or any other slender 

 and opake substance, and hold the paper sideways to the 

 window, two shadows are formed, the one blue from the 

 candle, the other brown from the daylight. Now this brown 

 shadow can be changed to an orange by approximating the 

 candle ; at a yet nearer distance the orange becomes a perfect 

 yellow, and when very close the colour entirely disappears, 

 or the light of the shadow becomes rarefied into perfect trans- 

 parency. The blue in like manner undergoes rarefaction 

 and change of colour, from a blue to a purple ; and when the 

 candle is very near the coil of paper, the shadow becomes black, 

 because then there is but one light, that of the sun. To argue 

 that the shadow is a mere privation, would be to say that 

 brown, orange and yellow, blue and purple were privations : 



