30 Prof. R. Threlfall on the Electrical 



(5 hours) for the gas to flow in. It will be convenient here 

 to describe the tube employed, though the dimensions given 

 are taken from a later experiment, which was supposed to be 

 a repetition of the one in question ; the dimensions of this tube 

 may therefore be in error by a few per cent., but the actual 

 dimensions are not of much importance. The ends of the tube 

 dip down into mercury cups (not shown) so as to protect the 

 part where the wires enter from leakage. The mercury pocket 

 in the tube which forms the subject of the diagram (fig. 2) is 

 attached to the spark-tube by means of a capillary tube 14 

 centim. long. 



This capillary tube was the finest piece of thermometer 

 tubing 1 actually happened to have by me, and was, as I should 

 imagine, about * 1 millim. in diameter *. In preparing the tube 

 after the final filling, the inlet at A is fused off, and when 

 the proper pressure is reached the pump connexion at B 

 is sealed, and afterwards, in two or three hours' time, the con- 

 necting tube of the mercury gauge. 



September 10-19. These days were spent in abortive 

 attempts to get tubes of the pattern described to submit 

 to having their electrodes white-hot for two or three hours 

 without fracturing the glass. 



September 19. Tube 5, as described, sealed off at 8' 7 

 millim. The mercury was put in hot through a side-tube, 

 but was not boiled in situ in order to avoid getting vapour 

 into the spark-tube. In order to avoid the presence of air- 

 bubbles between the mercury and the glass, the tube at G was at 

 first about 8 centim. long, and was drawn down to a fine point, 

 where it was fused off. The apparatus was then filled with 

 nitrogen and well heated when containing a nitrogen vacuum, 

 especial care being devoted to the mercury pocket. Nitrogen 

 was then admitted at more than atmospheric pressure, and 

 finally the end of the tube was broken off near and the 

 mercury run in as quickly as possible from a fine funnel 

 reaching to the bottom of the pocket. The tube at C was then 

 sealed, and the mercury warmed again to perhaps 100° C. and 

 the visible bubbles displaced, the tube being of course vacuous. 



Sept. 21. The mercury in the gauge was accurately at the 

 same height in both limbs. Sparked for five hours with the 

 best kind of sparks ; no effect. 



Sept. 22. Sparked four hours with an air-break in circuit 

 just sufficient to stop the spark but not the brush-discharge 

 across the air-break. It was noticed that luminosity went 



* This capillary tube was part of the later arrangements. In the tube 

 under discussion its place was taken by a tube 2 millim. in diameter and 

 only about 6 centim. long. 



