1901.] Effect of Light and Electric Radiation on Matter. 155 



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 pro- 

 duce 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 ex- 

 ternal 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 CS 2 , 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 mea- 

 suring 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 under- 

 gone by the strained substance, or the method of photographic de- 

 velopment. 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. 



