of Mixtures of Sulphur and Carbon. 351 



graphite and other forms of carbon is diminished by heat; and 

 it is also commonly believed that a rise of temperature inva- 

 riably causes a decrease in the resistance of insulators such as 

 sulphur. The compound of sulphur and carbon with which 

 we have to do is certainly only a mechanical mixture (for no 

 chemical combination could be formed at a temperature of 

 150° C, which is never exceeded in making the preparation); 

 yet the effect which heat produced upon it is exactly opposite 

 to that produced upon each of its constituents. 



Some experiments were made in order to determine the 

 effect of heat upon the resistance of carbon and sulphur sepa- 

 rately ; but, although in the case of sulphur some unexpected 

 results were obtained, nothing whatever was observed which 

 would per se account for the variations of resistance in the 

 sulphur and carbon mixtures,, 



I believe the true explanation to be this : — The mixture does 

 not consist of a uniform structureless mass of sulphur having 

 particles of carbon imbedded in and completely surrounded 

 by it. It is in fact an aggregation of little crystals of sulphur, 

 with carbon packed between them like mortar between bricks. 

 The conduction thus takes place entirely through the carbon 

 particles, which may be considered as extending in a series of 

 chains from end to end of tho mass. Under the influence of 

 heat, both the sulphur and the carbon expand ; but the expan- 

 sion of the sulphur is nearly ten times as great as that of the 

 carbon, the net result being that the carbon particles are 

 drawn apart, and have fewer points of contact with each other. 

 The number of complete chains is thus diminished, and the 

 resistance of the mass consequently increased. 



It is stated in Balfour Stewart's ( Treatise on Heat,' upon 

 the authority of Kopp, that sulphur, after being heated to a 

 certain temperature, contracts instead of expanding. This 

 would account for the fact that, after attaining a certain tem- 

 perature, the resistance of the mixture begins once more to 

 decrease; for the carbon particles would, when the sulphur 

 contracts, be brought together again. 



Several experiments corroborate this explanation of the 

 variation of the resistance. Thus a mixture was made con- 

 taining 3 parts of shell-lac to 2 of graphite. Though the 

 proportion of carbon was larger than in the sulphur experi- 

 ments, its resistance was found, as was expected, to be infi- 

 nite; for the structureless shell-lac penetrated between and 

 completely surrounded the carbon particles. A mixture of 

 paraffin and graphite gave the same result. 



A short rod of the usual sulphur-and-graphite compound (20 

 parts to 9) was fitted with platinun>wire electrodes; and its 



