ACTION OE LIGHT ON SELENIUM. 
321 
be referred to by their numbers, and, as it would not be desirable to have to describe 
each one of them whenever it is mentioned, we shall give here a brief account of the 
nature of each of the specimens which will be referred to hereafter. 
Mr. Willoughby Smith’s plate, which will be referred to as W. S., has been already 
described. 
No. 2 is a plate of selenium made up according to Dr. Siemens’s method. It was first 
heated in a water bath up to 100° C., kept at that temperature for some hours, and then 
cooled slowly. After this its resistance at ordinary temperatures was about 120 megohms. 
At a subsequent period it was heated in a paraffin bath up to 205° C., kept at that 
temperature for 3 hours, and then cooled gradually. The next morning its resistance 
at a temperature of 9 0- 5 C. was '402 of a megohm. 
No. 3 is a plate exactly similar to No. 2, and its history is almost identically the 
same. 
No. 4 is a plate similar in construction to Nos. 2 and 3. Its resistance before 
annealing was so great that we could not measure it. It was then placed in a paraffin 
bath, its electrodes being all the while connected with the usual arrangement for 
measuring its resistance. As the temperature rose its resistance gradually diminished 
until at 100° C. it was about 216 megohms. It was kept at this temperature for 2^ hours ; 
but this did not seem to have permanently affected it, as, on cooling it slowly, its 
resistance gradually increased again, so that at 55° C. it was 11 • 6 megohms, and when 
thoroughly cool its resistance was 120 megohms. The next day it was heated up to 
140° C., kept at that temperature for 3 hours, and then gradually cooled ; this had a 
permanent effect in reducing its resistance, which at a temperature of 9° C. is now about 
•193 megohm. 
No. 7 is a small plate of selenium whose platinum electrodes pass through corks at 
the end of a piece of glass tube. A length of about three millims. at the end of a piece 
of platinum wire was turned round at right angles, then two such pieces were laid upon 
a plate of talc, a drop of molten selenium let fall upon them, and then pressed down by 
another plate of talc. This plate was heated for 4 hours in a hot-air bath up to 110° C., 
and then cooled gradually. Its resistance at ordinary temperatures is now about ’585 
of a megohm. 
No. 8 is a plate made up at the same time and in the same way as No. 7. Its resistance 
at ordinary temperatures is about '213 of a megohm. This plate had previously been 
raised to 210° C., but not cooled sufficiently slowly. 
No. 10 is another plate prepared at the same time and in the same way as Nos. 7 & 8. 
The annealing was, however, not very successful in this case, its resistance at ordinary 
temperatures being about 3’2 megohms. 
In making up the remaining 15 specimens no talc has been employed. 
No. 11 is a small piece, about 4 millims. long, of a stick of selenium made up after 
the manner described on page 320. Its resistance at the ordinary temperature of the 
air is about - 0143 of a megohm. 
2 z 
MD CCCLXXVII . 
