Page:Collected Physical Papers.djvu/184

164 property of the substance. As a familiar instance phosphorous is changed from the yellow to the red variety by the action of visible radiation. After the allotropic transformation into the red variety, phosphorus has become chemically less active, insoluble in CS2, and of higher specific gravity. Similarly its other properties, such as its elasticity, its position in the voltaic series, its electric conductivity, etc., undergo a corresponding modification. An identical molecular phenomenon, seen from different aspects, may thus appear to be diverse. Looking from an electric point of view it is found that the conductivity of red phosphorus is greater than that of the yellow variety. It is thus possible to measure the induced molecular change by measuring the correlated variation of any of the properties described above. The choice of a particular method will be governed by special convenience under given conditions.

It would thus be possible to detect the effect of molecular strain induced by visible or invisible radiation by the following more or less sensitive methods. It is to be borne in mind that the effect of radiation is almost confined to the skin or outer layer of the substance.

(1) Method depending on the variation of the adhesive or cohesive power of a substance, e.g., in a daguerreotype plate, the mercury vapour adheres by preference to the light-impressed portions only. Images may in a similar manner be developed by water vapour.

(2) Method depending on the variation of chemical activity undergone by the strained substance, or the method of photographic development. The acted and unacted portions are differ-