482 SCIENCE PROGRESS 



is a property which has been detected only in solid bodies, i.e. 

 bodies which possess elasticity other than bulk elasticity, and 

 hence are capable of exhibiting elastic after-effects. It seems 

 reasonable to expect, therefore, that something more than a mere 

 analogy will be found between the two properties, and that a 

 real connection exists. 



Finally, it has been recently found by N. A. Hesehus 1 that the 

 changes of resistance in a selenium cell resemble the variations 

 in strain referred to in detail above. A selenium cell has different 

 resistances according to the intensity of the light to which it is 

 exposed, but the change of resistance is not immediately com- 

 plete when the light intensity is altered ; in other words, the 

 resistance is a function both of the intensity and the time. Thus, 

 when the cell, after being exposed to light, is put in complete 

 darkness, its resistance increases very rapidly at first, but more 

 and more slowly as time elapses, and a considerable period 

 of time must intervene before the resistance reaches its final 

 constant value. 



No doubt further instances could be enumerated — instances 

 in which the value of one physical quantity is dependent upon 

 the value of a second quantity and also upon time — all of which 

 make wider and wider the field for research on elastic over-strain 

 in its more extended sense. 



1 N. A. Hesehus, Journal de la Soc. Phys. Chew, russe, 1906. 



