and of certain Liquids on the Surface of Water. 387 



upon the surface of the water ; the ether must not touch the 

 film, or it will destroy it, while if the ether be taken up by 

 a body that has not a pointed or rounded termination the 

 rings will not be formed, and there will only result a series of 

 concentric figures, depending for form upon that of the end of 

 the substance held over the film. Instead of ether we may 

 employ solution of ammoniacal gas, pyroligneous ether, alcohol, 

 or naphtha. Strong ammonia sometimes breaks up the film and 

 scatters it over the surface of the water." 



A turpentine film, which is at first colourless, soon begins to 

 display colour as the film becomes thinner by evaporation — a 

 remark that does not apply to a film formed by a drop of the 

 fixed oils of olive, rape, castor, nut, &c. 



Films that exhibit some of the most splendid colours of 

 thin plates may be formed from a drop of balsam of Peru or a 

 turpentine varnish, such as copal varnish, carriage- varnish, 

 gold size, black japan, &c. If the ether be held over one of these 

 coloured films, " we immediately get a series of rings as before, 

 quite independent of the large rings which occupy the surface 

 of the water. That part of the film subjected to the action of 

 the ether no longer forms a portion of the original film, but is 

 subject to the systematic arrangement which the ether produces. 

 A film showing the colours of the 5th, 6th, and 7th orders only 

 is, at that particular part which is subjected to the action of the 

 ethereal vapour, made sufficiently thin to exhibit the rings of the 

 1st, 2nd, 3rd, and 4th orders, or of the 2nd and 3rd, or of the 

 3rd and 4th, — all depending on the quantity of ether taken up, 

 the rate of evaporation, and the proximity of the ether to the 

 film — in other words, on the comparative thinness of the film. 

 Should the drop of oil &c. form a lens instead of a film, the 

 ether vapour will drive it with great energy over the surface of 

 the water." 



If the water be dusted over with a dry powder, the vapours 

 of ether &c. will powerfully repel the particles. " Turpentine 

 and some of the volatile oils produce a similar effect upon 

 powders ; and one volatile oil frequently repels a film formed by 

 another." 



The above details refer to the repulsion of films. "A strong 

 and decided attraction may be exhibited by presenting to the 

 film a drop of an acid of low boiling-point, such as nitric or 

 pyroligneous acid. By such means an oil-film of the size of a 

 crown-piece may be reduced to the size of a sixpence ; and a film 

 exhibiting colour will have all colour destroyed by the proximity 

 of the acid, whose influence is to thicken the film. Sulphuret 

 of carbon and an aqueous solution of chlorine produce a similar 

 result" (pp. 545-549). 



2D2 



