2B0 Prof. Minchin's Experiments in Photoelectricity, 



rapidly smeared over the aluminium plate. This selenium 

 layer should be of uniform thickness, and the thickness must 

 be neither very great nor very small. When the layer is 

 deposited, the aluminium plate is quickly removed from 

 the porcelain plate by the forceps and rapidly moved 

 up and down in the air for a few seconds, the gas- 

 flame being, at the same time, removed from under the 

 porcelain plate. Both plates having now become slightly 

 cooler, the aluminium plate is replaced on a comparatively 

 cool part of the porcelain plate, and any tendency of the 

 selenium to become liquid is checked by blowing over its 

 surface. Working the gas-flame now rapidly backwards 

 and forwards under the porcelain plate and occasionally 

 blowing over the selenium surface, a series of changes — very 

 much resembling those presented in the preparation of a 

 sensitive tin surface — are observed. The appearances are as 

 follows : — 



1. The originally jet-black selenium surface gradually 

 assumes a bluish-white appearance. 



2. As the process is continued, this latter surface becomes 

 a grey which may be of several shades. It may be a light 

 grey, or a grey with a violet tinge, or a grey with some 

 glossy spots or streaks. None of these surfaces is to be 

 accepted. They are the final forms which most readily pre- 

 sent themselves ; and when they do, the selenium must be 

 melted afresh and again spread over the aluminium plate — 

 the whole process being repeated with its gradual heatings 

 and coolings, until finally — 



3. The surface of the selenium assumes a very dark brown 

 colour. 



This is the most sensitive surface that can be obtained. 

 At first, accepting too literally the statement that " the grey 

 modification of selenium is the sensitive one,'"' I accepted 

 every plate which finally assumed a grey appearance, and 

 constructed a large number of cells for a battery. By 

 accident, however, a plate with the brownish colour was 

 formed, and it proved to be so much superior to the others, 

 that they were all rejected. 



The glossy spots and streaks which sometimes exist on the 

 grey surfaces are, I think, due to an indefinitely thin layer of 

 the black selenium which has escaped the necessary transform- 

 ation ; and to observe them, it is well to look at the plate 

 almost in the plane of its surface. The dark brown surface is 

 devoid of them, and is in appearance quite homogeneous. 



When the plate has assumed this appearance, it may be 

 screened from light and left on the porcelain plate to get 



