IOWA ACADEMY OP SCIENCE 
269 
But thus far I have not been able to detect any change of resistance in 
crystal contacts because of any direct or even indirect action of light. 
At present my hope is that I may find some crystal or crystals that 
change form or volume under the action of light. If such are packed 
among small crystals that show resistance because of the surface con- 
tacts, then we should have a variation of the resistance of the mix- 
ture by illumination by virtue of the change of pressure. Theoretically 
we should be able to .produce in this way an approximate working 
model of a selenium cell. 
It may be that W. S. Graepenberg has duplicated the action of the 
selenium cell in his antimonite, a mixture of antimony and sulphur. 
By properly cleaving these crystals he is able to produce a light-sensitive 
cell. The resistance is apparently like that in ordinary crystal contacts 
in that the resistance exists between a gold foil and the surface of the. 
crystal. The crystal is illuminated through the gold foil. This cell 
structure is about as sensitive to light as the selenium cell and it seems 
to have all the properties common to both selenium and crystal con- 
tacts. But so far as I am at present able to' judge, it is only surmised 
that antimonite crystals bridge over the gap. If we should find definitely 
a crystal or crystal mixture which was known to be light-sensitive like 
selenium and at the same time know that its resistance depends on 
contact surfaces, we should have a new and very simple approach for 
explaining the complex behavior of selenium. A curve showing how 
the resistance of the antimonite cell varies with potential difference is 
given in fig. 3. This is strikingly similar to such curves for selenium 
and elementary crystal contacts. 
