2Cy Prof. R. Threlfall on the Electrical 



behind it that there was a slight appearance of a bronze- 

 coloured fog in the tube near the mercury-pocket. 



June 16. A strong discharge sent through the tube so that 

 it became warm to the hand, say 40° C. ; the discharge was 

 kept on for three hours. 



June 18. The temperature again being the same as when 

 the tube was sealed, it was noted that the effect appeared to 

 have increased by about '2 millim. in spite of the strong 

 discharge. Two " gallon " Leyden jars were then included 

 in the circuit provided with an air-gap, and parallel to the 

 spark-tube. During two hours' sparking the effect went on 

 increasing, and the bronze-coloured stain on the glass grew 

 deeper near the drops of mercury till it became a black 

 mirror. 



This was of course partly due to the volatilization of the 

 mercury near the drops. The effect bad increased to about 

 7 millim. by the next morning, when the temperatures had 

 become uniform. 



June 19. Sparked, say, 5 hours. Effect on cooling was 

 now 7*3 millim., indicating that only about 1*2 millim. of gas 

 was left in the tube. 



June 20. The mirror of mercury compound, which had now 

 become very black about the narrow tube and in the part of 

 the spark-tube near it, was heated with the Bunsen flame. 

 After some heating, say at about 250° C, the film began to 

 crepitate audibly and emit flashes and sparks of blue flame. 

 The deposit turned brownish as the temperature rose, the 

 sides became indistinct, and finally the flash and crackling 

 noise came on and the film vanished, leaving a clear bright 

 tube. The process was repeated till the mirror of mercury 

 had been chased all over the tube ; and even then the gauge 

 only came back far enough to show a pressure-difference 

 of 4*2 millim. in the same direction as before — i. e. it only 

 came back by 3'1 millim. The ends of the tube, where the 

 the electrodes were fused in, were not heated ; and this may 

 have accounted for the persistency of the diminution of 

 pressure. In order to test this the tube was removed from 

 the mercury cups and placed in an extemporized air-bath 

 made out of a kerosene tin. Several thermometers were 

 ranged alongside the tube, and the whole apparatus was 

 covered by an asbestos cloth. The bath was heated by a 

 Bunsen burner. A good many rather elaborate readings of 

 temperature and pressure were taken on June 20, the 

 temperature rising to about 170° C. in the middle of the 

 tube and to a few degrees less at the ends. These readings 

 turned out to have no significance, in consequence of a doubt 



