Some experiments made by Talbot and Lord Rayleigh have been repeated and extended. A film was prepared by evaporating a solution to dryness on glass, then fusing the salt and letting it cool. In the case of potassium nitrate and potassium chlorate, when the film was placed under a microscope and between crossed nicols, Talbot and Rayleigh had observed irregularly distributed patches varying from dark to light, and variously colored. It is now found that silver nitrate and benzoic acid also show the effect. The intensity diminishes with time but the variations can still be observed after six months. A film formed by evaporating a binary solution of a mixture of any pair of the following: alum, boric acid, borax and citric acid, also showed the effect although films containing only one constituent were inactive. The effect is doubtless the result of mechanical strains developed on cooling the film, similar to those obtained when glass is chilled from above its softening point.

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