414 Some Eectrical Experiments with Crystalline Selenium. 



It appears from the above, that the agreement between the 

 calculated values representing the change, on the supposition 

 that it is due to a decrement of resistance, is much greater 

 than that between the values calculated on the supposition 

 that it is due to an electromotive force set up in the selenium 

 in the same direction as the battery current. 



There is much experimental work to be done yet with 

 selenium before any theory of its behaviour can be advanced 

 with confidence. So far, the experiments seem to suggest the 

 suspicion that light causes a modification of the surface tension 

 of selenium, possibly an expansion of the crystalline surface. 

 The superficial crystals expanding and pushing against each 

 other, so as to improve the doubtful points of contact previ- 

 ously existing between them, may account for the observed 

 increase of conductivity in the light. Such a superficial ex- 

 pansion would probably be occasioned by heat ; and this 

 heat on the extreme surface might account for the alteration, 

 by light, of the potential of the selenium plate, when made up 

 in the form of a galvanic cell, being in the same direction as 

 the alteration of potential produced by the direct application 

 of heat when the selenium is in the dark. It might also 

 account for the decreasing sensibility of the selenium by con- 

 tinued exposure to light, the superficial heat penetrating into 

 the interior and relieving the state of tension of the surface. 



I apprehend that the superficial atoms of any body, which 

 are bounded on one side only by similar atoms, and on the 

 other side by the medium in which the body is immersed, may 

 be capable of assuming vibrations of different periods to those 

 which the atoms underneath the surface can assume. It may 

 also be that the luminous rays striking upon the superficial 

 molecules of selenium, impart a vibration to them of a slower 

 period than those of the exciting waves, and which corre- 

 sponds nearer to the period of the heat rays. 



To return, however, to the object with which this inquiry 

 was undertaken, viz. the production of constant resistances 

 for measuring-purposes, it is evident that selenium is, from 

 its peculiar nature, a very unsuitable material. In its amor- 

 phous state it is dielectric ; and in its imperfectly crystalline 

 state its character seems to be that of a dielectric more or less 

 charged with conducting crystals. 



This character it probably never entirely loses, even when 

 crystallized as far as it can be ; and to this fact is probably 

 due, in a great measure, its peculiar behaviour. 



In the light it would of course be utterly useless for 

 measuring-purposes, whilst in the dark the apparent resist- 

 ance of its junctions with the conducting wires changes, not 



