tivity change is not always permanent, since several substances are known which quickly recover and attain their original conductivity on the cessation of radiation, as if a force of restitution were called forth to restore them to their original condition.
I was thus led to suppose that the effect of radiation is to produce a state of molecular strain. Evidences will presently be adduced which it is hoped will furnish proofs as to the correctness of this view.
If a substance is molecularly distorted by the action of an external agent, we may naturally expect that there would be produced changes in the physico-chemical property of the substance. As a familiar example, take the case of phosphorus changed from the yellow to the red variety by the action of visible radiation. We find that in the allotropic condition of red, the phosphorus has become less active chemically, 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, &c., are likely to undergo a corresponding modification. The same molecular phenomenon, seen from different aspects, may thus appear to be diverse. Looking from an electric point of view we do find that the conductivity of red phosphorus is greater than that of the yellow variety. We thus see the possibility of measuring the 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.
If the above view is correct then it would be possible to detect the effect of molecular strain due to visible or invisible radiation by the following more or less delicate 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 in preference to the light-impressed portions only. Images may in a similar manner be developed by water vapour. Under the action of electric radiation, particles of certain metals are known to stick together. But this is by no means universal.
(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 differently attacked by the developer. The action is not altogether independent of the effect described below.
(3.) Method depending on the variation of electric potential, by which an E.M.F. is produced between the acted and unacted portions of a substance originally iso-electric.
(4.) Method depending on the conductivity variation produced by the strain.
