of Mixtures of Sulphur and Carbon. 349 



upon another occasion, with special reference to the paper in 

 support of the opposite view read last year by Dr. Moser. 



The second point in which the sulphur rod appeared to 

 differ from selenium, was in the direction of the change which 

 the radiation of the gas-flame produced, the galvanometer 

 showing that its resistance was increased, instead of being 

 diminished as might have been expected. Here, however, 

 upon the supposition that the effect is due to heating, the 

 behaviour of the sulphur is really similar to that of selenium ; 

 for my experiments (agreeing with those of Prof. Adams) 

 show that at ordinary temperatures slight heating is always 

 accompanied by considerably increased resistance. 



After numerous trials, in order to ascertain what propor- 

 tions of sulphur and graphite yielded the greatest sensitive- 

 ness to heat, it appeared that a mixture containing 20 parts 

 by weight of sulphur to 9 of graphite was the most suitable. 

 It was also found to be more sensitive when cooled rather 

 quickly than when cooled slowly, though its specific resist- 

 ance (which is always high) was generally lower in the latter 

 case. 



In order that the peculiar property of the substance may be 

 exhibited in the most effective manner, it is necessary to 

 arrange it in thin films, so that a large surface relatively to the 

 bulk may be exposed to the action of radiation. This was at 

 first done by spreading it as thinly and evenly as possible 

 upon plates of mica having tinfoil electrodes at each end. An 

 objection to this method, however, was the enormously high 

 resistance which it involved, amounting sometimes to several 

 hundred thousand ohms. I therefore adopted the device which 

 I generally use in the construction of selenium cells, and which 

 is a simple modification of a plan originally proposed by Dr. 

 Werner Siemens. 



Two wires, preferably of platinum, are wound parallel to 

 each other, and very close together, around a slip of mica, 

 care being taken that the wires do not touch each other at 

 any point, A film of the melted mixture is spread evenly 

 over one surface of the mica ; and the wire electrodes are thus 

 connected with each other through half their entire length by 

 a series of very narrow strips of the sulphur mixture. 



The resistance of a sulphur "cell" constructed in this 

 manner was 9100 ohms at a temperature of 14° C. The cell 

 was slowly heated in an air-bath, and the resistance measured 

 as accurately as possible at almost every degree. The follow- 

 ing table sufficiently indicates the rate at which the resistance 

 increased. 



