196 Mr. H. H. Poole on the Temperature Variation 



rapidly with the temperature, being approximately pro- 

 portional to the 16th power o£ the absolute temperature, 

 while b was approximately constant ; but as the temperature 

 only ranged from 0° C. to 47° C. no very definite conclusions 

 could be drawn from the results." 



Several other forms of condenser were then tried, the con- 

 ductors being either well-ground brass surfaces or liquids 

 such as mercury or glycerine. None of these were very 

 satisfactory, as the solid surfaces did not make sufficiently 

 good contact with the mica to ensure consistent results, 

 while, in the case of the liquids, trouble was experienced 

 with the insulating enamel used for cementing the glass 

 vessel containing the liquid to the mica. These experiments, 

 however, clearly showed that a varied very rapidly with the 

 temperature, and also was very different for different spe- 

 cimens of mica, while b was approximately independent of 

 the temperature, and had about the same value for all the 

 different specimens used. 



Finally, the simple expedient of silvering the mica was 

 adopted and proved satisfactory. The silvering was done 

 by Carey Lea's process, the so-called gold-coloured silver 

 being used *. The silvering mixture was applied with a 

 brush, and a very good coating was easily obtained which 



Fig-. 1. 



was quite a good enough conductor for the purpose. As the 

 unsilvered parts of the mica surfaces had to be left bare, 

 instead of being coated with shellac, which would not have 

 stood the high temperatures, trouble was anticipated from the 

 brush discharge which had been noticed before ; this has the 

 effect of increasing the effective area. An attempt was made 

 to reduce this source of error by giving the condenser the 

 form shown in section in fig. 1. Here the thickness of the 



* See ' Chemical News.' vol. lix. p. 54 (1889). I am very much 

 indebted to Prof. E. A. Werner, F.I.C., for suggesting this process and 

 preparing the mixture for me. 



