136 
PROFESSOR R. THRELFALL AND SIR. J. H. D. BREARLET 
The question of surface conductivity is finally settled by this film ; we got just the 
same results whether the quartz rods were in or not, or whether we kept the film in 
the box or exposed it to damp air, or even breathed on the surface of the sulphur 
between the dish and the upper plate. Using the excellent condenser of Messrs. 
Clark, Muirhead, and Co., we were enabled to make the capacity comparisons with 
greater accuracy, and are thus in a position to assign an approximate value to the 
temperature coefficient of the specific inductive capacity—which turns out to be 
positive, and of the approximate value I'D X 10 -6 per degree Centigrade. 
The increase in specific resistance suggests that perhaps even now we are not using 
a material of sufficient purity. In favour of this view we may state that the film was 
not quite evenly coloured, some portions appearing of a browner shade than others. 
On the other hand we know of no reason to suspect any impurity, even the action of 
dust having been carefully guarded against, and the sulphur itself being admirably 
pure. It was perhaps, however, insufficiently exhausted of gas. On the other hand, 
we know that when annealed the resistance of sulphur as we used it is much higher, 
and it is difficult to credit a supposed impurity with the property of itself undergoing 
a change of constitution at the same temperature as that at which sulphur anneals. 
The observations show a very definite reduction of resistance as following a rise of 
temperature to 80° C., and this persists to some extent on cooling. We noticed this 
before. We know from a special experiment, in which some finely-powdered sulphur 
was heated to 80° C. for an hour, that the percentage content of insoluble sulphur is 
thereby diminished. For instance, in a very careful experiment made on pure, finely- 
powdered sulphur in quantities of about ten grams, it appeared that the percentage 
content of insoluble sulphur was 4'01 before annealing, and 3'89 after annealing for 
several hours. A critical examination showed that this was entirely outside the limits 
of any possible experimental error. The annealing at 90° C. had been previously 
observed. We provisionally attribute the decrease of resistance to this partial 
annealing, which therefore tends to show that the conductivity of mixed sulphur has 
a maximum value somewhere between the 0 per cent, and 5 per cent, content of 
insoluble sulphur. The explanation of the temperature curve is, however, not by 
any means obvious, although it seems safe to draw the following conclusion, 
assuming that the temperature lag is inappreciable. The conclusion is that there 
must be at least two causes in operation influencing the resistance. One of these is 
probably the temperature per se, and the other the rate of transformation of insoluble 
into soluble sulphur, which is probably a complex phenomenon. The complexity may 
possibly ariseWrom the change itself not being direct, the amorphous sulphur being 
first converted into a less stable form. There is also evidence, which will be given 
later, that at least two varieties of insoluble sulphur coexist in a rapidly cooled film. 
On referring to the table it will be evident that all the phenomena are similar to 
those formerly observed, but on a smaller scale, owing to the conduction being less. 
All the effects of time, of reversal of voltage, &c., were observed as before. We 
