hs 
Cu 
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PROCEEDINGS OF SECTION A. 
SECTION II.—ACTION OF LIGHT ON THE CONDUCTIVITY. OF 
SULPHUR. 
The action of light in reducing the resistance of selenium is 
well known. Dr. Monckman (Phil. Proc., 1889) considers that 
he has observed a similar effect with sulphur, but the observations 
chronicled by him leave considerable doubt on the question. Of 
his three sets of observations, the first points to a considerably 
reduced resistance in the light; the second and third to a slight 
increase of resistance. The conductivity varies considerably from 
time to time. Dr. Monckman’s experiments can hardly, there- 
fore, be considered conclusive, even assuming the purity of his 
material. 
Some experiments have been made by us to determine whether 
light really has any appreciable effect upon the conductivity of 
sulphur. 
The first question to be considered was as to what molecular 
modification should be used. Sulphur, in the soluble state, is at 
the present limit of experimental method practically a perfect 
non-conductor. Pure insoluble sulphur on the other hand cannot 
be obtained in a film suitable for experiment. The only course 
was, therefore, to use a mixture of soluble and insoluble sulphur, 
for which some small conductivity has been proved. This being 
so, it was obviously desirable to obtain the sulphur in the state 
of greatest conductivity, and to this end the sulphur was prepared 
with as great a proportion of insoluble sulphur as possible.* 
Analyses made with two cells used, gave 21°35 and 14-96 as the 
percentages of insoluble sulphur. The composition of the other 
cells was probably within these limits. As regards quality, 
the cells were made*from roll sulphur twice distilled, one or two, 
however, being made from sulphur that had only been once 
distilled. 
Various experiments were made to determine what form of 
sulphur resistance cell was most suitable, the following form 
being finally adopted. A rectangular slab of sulphur was taken, 
about 6 cm. long by 3 cm. broad, and from 3 to 4 mm. thick. 
This was wound with two parallel spirals of thin platinum wire, 
which were nowhere allowed to touch, but were wound as closely 
as possible, the average distance between the spirals being a little 
over 1 millimetre for most of the cells. After winding, a hot 
glass rod was passed over the wire, which was thus pressed into 
and covered with a thin film of sulphur. A large conducting 
surface of sulphur is obtained in this way, which can easily be 
exposed to light. 
*It was thought at this time that the conductivity would be found to increase with the 
proportion of insoluble sulphur pregent. 
