certain Vapours on Films, fyc. 119 



vapour being very heavy, falls down upon the excessively at- 

 tenuated film, whose thickness must be measured by millionths 

 of an inch. This heavy repulsive vapour then sweeps aside 

 the film in a regular manner, producing in some cases a large 

 central opening, and then a thinning off of the film, suf- 

 ficient to allow of the interference of the light required for the 

 phenomena of coloured rings. This descending vapour, more- 

 over, meets with an ascending vapour from the film, and the 

 contact of the two produces further complications in the way of 

 chemical and electrical effects which I will not ask you to con- 

 sider at present. I will only remark, first, that the electrical 

 condition of these vapours is very decided, and must be con- 

 sidered before the explanation of these phenomena is at all 

 complete; and secondly, that during solution, as of a salt in 

 water, current electricity is (as I have reason to believe) largely 

 concerned in the action and in the motions of the solid in the 

 solvent. But my immediate point is with the mechanical action 

 of this repulsive vapour from the wet sponge upon the film, the 

 light powders, and the mobile discs of creosote, &c. There is 

 a mechanical action about this vapour which goes some way to 

 explain the production of the rings. A stream of gas from a 

 gas-bottle produces them, and, what is equally remarkable, the 

 vapour of oil of turpentine will repel a turpentine film. A drop 

 of turpentine on water forms a good film, often at first without 

 colour, but as it evaporates it displays colour. As soon, how- 

 ever, as the film is formed, if a sponge dipped into the same 

 bottle from which the drop was taken be held over the film, it 

 will spread out into very beautiful rings. 



But now let us consider the influence of attraction. Ad- 

 mitting for the moment that water becomes saturated with one- 

 eighth of its bulk of ether, it will be found that there is a very 

 strong attraction between ether and its vapour for water up to 

 this point. The water quickly becomes saturated, but the com- 

 bination up to saturation is very energetic, especially at first. 

 The great density of ether-vapour also assists the attractive force 

 of the water ; it falls down, the water seizes it, and disperses 

 any dust or powder that may be in its way. If a film of oil 

 or varnish be interposed, it begins to dissolve that, and thins 

 it out to the gradually decreasing thicknesses necessary to the 

 display of Newton's rings, often making a complete perforation, 

 half an inch in diameter, in the film to get to the water, and 

 showing the rings of colour at the inner edge of this perfo- 

 ration. 



That this explanation is likely to be true the following ex- 

 periment will prove: — Seven parts of water and one part of 



