600 Hon. R. J. Strutt on the Electrical 



presented the ordinary appearance of a metal at a yellow, or 

 nearly yellow, heat. When this stage was reached, the tube, 

 which was the strongest I was able to make, burst with a 

 loud explosion. Some of the fragments were examined, and 

 they showed a " conchoidal " fracture, along a diametral 

 plane of the tube. The quartz had ruptured abruptly under 

 the internal pressure. There was no sign of viscous yielding. 



Of those elements which behave electrically as undoubted 

 metals, mercury is the most volatile. The next is arsenic, 

 and it was thought desirable to try to observe its critical 

 phenomena. Some powdered arsenic was hermetically sealed 

 up in a strong quartz tube, and the tube heated as before. 

 At a dull red heat the arsenic melted to a silvery liquid. As 

 is well known, it will not melt at ordinary pressures, but 

 passes from the solid to the gaseous condition. 



The vapour began at once to show a strong yellow absorp- 

 tion tint. I satisfied myself that the colour was due to 

 absorption, and not to radiation, by observing it against a 

 background of bright sky. The vapour of arsenic begins to 

 show (selective) absorption of light at a very much lower 

 temperature than does mercury. So that it sooner approaches 

 the opaque quality of the liquid metal. There seemed, 

 therefore, to be a chance that its critical temperature might 

 be reached. This hope was disappointed. The tube con- 

 taining arsenic was heated (all over) in a large blowpipe- 

 flame, and the temperature raised by enriching the air-blast 

 with gradually increasing proportions of oxygen. The liquid 

 and vapour began to look somewhat alike, but with close 

 observation could easily be distinguished. Finally, the quartz 

 began to yield, viscously in this case, and the experiment 

 could be carried no further. 



§ 3. Observations on the Electrical Resistance of Hot 

 Mercury and its Vapour, 



Although the above experiments made it unlikely that the 

 critical temperature of mercury could be reached in quartz 

 tubes, it remained possible that some indication of the changes 

 which the electrical resistance w^ould undergo at that tem- 

 perature might be got by observations at lower temperatures. 



To make a tight joint whereby a conducting wire can be 

 led into a quartz tube containing mercury, red hot, and at an 

 enormous pressure, seems at first sight a somewhat formidable 

 mechanical problem, the more so since it is not possible to 

 fuse platinum air-tight into quartz. 





