344 
PROFESSOR W. a. ADAMS AND MR. R. E. DAT ON THE 
there was a deflection of 7 divisions to the right, showing that the light gave rise 
to a current from platinum to selenium at the unmarked end. Hence in this plate of 
selenium sunlight produces a current from platinum to selenium at the illuminated end, 
but the marked is much more sensitive than the unmarked end. 
This agrees with previous experiments described in the paper, with the exception 
that the lime-light was not sufficiently powerful to give rise to a current when it was 
brought to a focus on the unmarked end. 
It will be seen from the first four experiments just described, the results of which 
entirely agree with our previous investigations, that the action of sunlight on the 
selenium produces a very singular effect when a current from a battery is passing 
through it ; in all cases it seems to assist the battery-current in whatever direction it is 
passing, so that the resistance of the selenium appears to be diminished ; and yet, when 
there is no battery-current, sunlight causes a current from platinum to selenium at the 
junction on which it falls. 
To Mr. Willoughby Smith’s plate, which is 1^ inch in length, heat was applied by 
holding each of the junctions in turn between the finger and thumb, the connecting 
wires being attached to a galvanometer. When the marked end was heated, a current 
was produced from the platinum to the selenium at the heated junction. When the 
unmarked end was heated, less effect was produced, but the current was from the 
platinum to selenium at the heated junction. 
The results of the experiments just described agree with the results of the investiga- 
tions of Dr. Matthiessen on the thermoelectric properties of selenium, which led him 
to place platinum above selenium in the thermoelectric scale. 
For the smaller pieces the following method of heating the junctions was adopted. 
A glass tube about 1 centim. in diameter was drawn out to a very fine point, and a 
source of heat, usually one or more Bunsen burners, placed underneath it, the fine point 
being placed opposite to and directed towards the junction to be heated; a blast of air 
was then sent into the large end of the tube, which became heated by the tube, and 
issued as a hot blast at the fine point where it fell upon the junction of selenium and 
platinum or upon any required point. On applying this method of heating to 
Mr. Willoughby Smith’s plate at the marked end a current was produced from plati- 
num to selenium, and on heating the unmarked end a current was produced from 
platinum to selenium at the unmarked end. Hence platinum stands above this piece 
of selenium in the thermoelectric scale. 
Other specimens of selenium were then tried in the same way, and, where it was 
possible, heat was also applied by holding the junction of platinum and selenium between 
the finger and thumb. 
In nearly all those pieces of selenium with which the investigations described in the 
early part, of this paper were carried on, a current was found to be produced at the 
heated junction from platinum to selenium, showing that the changes through which 
they had passed in the process of annealing had not raised these pieces of selenium 
