ACTION OF LIQ-HT ON SELENIUM. 
317 
of the beam, and, on exposing the selenium, no effect whatever was produced. This 
experiment was repeated several times with the same result. 
The plate of smoked rock-salt was then used alone with the same result, namely, of 
depriving the beam of all power of affecting the selenium. 
A solution of iodine in bisulphide of carbon, which appeared perfectly opaque to ordi- 
nary gaslight, Avas then placed in the track of the beam, and, after exposure for one 
minute, the diminution of resistance was represented by a deflection of 100 divisions on 
the galvanometer-scale. This appeared to contradict the results of the preceding 
experiments ; but, on looking through the solution directly at the carbon points, some 
red light was seen. 
The thickness of the solution was then increased, and it was noticed that the effect 
on the selenium was diminished in proportion as the solution became more and more 
opaque to the luminous rays. 
Hence we may conclude that heat-rays do not act powerfully in reducing the electrical 
resistance of selenium. 
The next point to be examined was as to how much of the effect produced on the 
electrical resistance of the selenium was due to the action of the chemical rays coming 
from the light to which it was exposed. 
For this purpose pieces of glass of various colours and of different thicknesses were inter- 
posed between the source of light and the selenium, and it was found, as a general 
result, that the amount of diminution of resistance on exposure depended on the intensity 
of the illuminating power of the light falling upon the selenium, and not on the presence 
or absence of the chemically active rays. 
In one series of experiments, when the source of light was a paraffin-lamp, the light 
was allowed, in the first experiment, to fall directly on to the selenium, and in the tAvo 
subsequent ones it was sent through a ruby and an orange glass respectively in order to 
absorb the chemical rays — the result being that, with air alone, the diminution of resist- 
ance was 3 - 86 per cent., while with the ruby and the orange glasses it was 2-96 and 
2 '6 3, and with a thick piece of green glass ‘7 per cent., the lessened effect being clearly 
due to the absorption of light by its passage through the glass. 
On another occasion the light was first sent through air, and then through a solution 
of terchloride of phosphorus, and the consequent diminution of resistance was 700 ohms 
in the case of air and 650 ohms with the terchloride of phosphorus. Where water was 
substituted for the terchloride of phosphorus, the effect was almost exactly the same. 
These experiments were repeated on several occasions, and the results obtained were 
in all cases similar to those we have just described. 
The effects of exposure to various parts of the solar spectrum, and also of the spectrum 
of the electric light, were examined, and it was found that the action on the selenium 
was greatest in the greenish-yellow and in the red portions of the spectrum, the violet 
and the ultra-red rays producing very little, if any, effect. 
