604 Hon. R. J. Strutt on the Electrical 



aggregate of* crystals, which were electrically continuous*. 

 To make contacts with the ends, the sealed extremities of the 

 limbs were ground off obliquely on a grindstone. They were 

 then plunged into a fusible metallic alloy//, contained in the 

 brass caps cc shown in the figure. This alloy is the same as 

 that introduced by Mr. E. H. Griffiths, It contracted 

 firmly round the quartz on solidifying, and made good con- 

 tact with the arsenic. 



On heating the top of the tube the arsenic melted in the 

 parts cc, and liquid portion rested on the solid thl, which 

 acted as a conducting cement. The top part b contained 

 arsenic vapour. 



This method of leading the current in and out proved 

 quite satisfactory ; the joints remained tight as long as was 

 desired. 



In an experiment the tube was joined up witli the battery- 

 cell and galvanometer previously mentioned. A deflexion 

 of 60 mms. was observed at a bright red heat, the highest 

 temperature attainable in a Herapath blowpipe. 



The length of the vapour-column was about 3 cms., and its 

 cross-section about 1 mm. square. Thus the specific re- 

 sistance of the saturated vapour at a bright red heat is about 

 10 4 ohms. 



The specific resistance of solid arsenic at 0° is about 

 3' 5 X 10~ 5 ohms. There are no data for exactly predicting 

 the resistance of the liquid above the melting point, but it 

 must be of the order of 10~ 5 ohms at 1000° C. 



Thus the resistance of the vapour is 10 9 times that of the 

 liquid at this temperature. 



It was thought interesting to determine whether or not 

 the vapour of arsenic obeyed Ohm's law. For this purpose 

 an E.M.F. of 400 volts was applied to the tube, and the 

 galvanometer shunted in a known manner so as to make the 

 deflexion suitable. The temperature was kept at a bright 

 red heat as before. It was not possible to keep the tempe- 

 rature very steady, but within the limits of accuracy of the 

 experiment the current observed was 200 times as much as 

 before when the E.M.F. was increased two hundred fold. 



§ 4. Conclusion. 



The results of the preceding reasoning and experiments 

 ma}' be summarized as follows : — 



(1) Mercury vapour is an insulator, while liquid mercury 



* Finely powdered arsenic appears to insulate perfectly, so far as can 

 "be observed by ordinary experiments with a galvanometer. 



