Conductivity of Metals and their Vapours. 599 



We have, then : — 



Critical Temperature of Mercury. 



From temperature coefficient of surface-tension 750° 



From the boiling-point 724° 



From the thermal expansion at moderate temperature .... 2700° 



It is evident from this table that the methods which are 

 successful in estimating the critical temperatures of ordinary 

 liquids entirely fail when it is attempted to apply them to a 

 metal. 



The fundamental difference between mercury and other 

 liquids, so far as critical phenomena are concerned, probably 

 lies in the very great density of the former. Few of the 

 ordinary liquids which have been investigated are so much 

 as 1*5 times the density of water. Mercury has 14 times 

 that density. Thus the vapour-density has to increase 

 enormously before it can compare with that of the liquid. 



It is, however, difficult to understand the entire failure of 

 the argument from increase of surface-tension. Probably a 

 careful investigation of the surface-tension of mercury up 

 to the highest temperatures practicable would yield interest- 

 ing results. 



I have attempted, but without success, to observe the 

 critical phenomena of mercury. The experiment was carried 

 out as follows : — 



A tube of quartz was built up according to the directions 

 given by Shenstone *. Its length was about 3 cms., internal 

 diameter 1 mm., external diameter about 7 mms. It was 

 provided with a handle of quartz rod, fused on at one end. 

 About two-thirds of the length were filled with mercury, and 

 the end was hermetically sealed, taking care to make the sealed 

 end as strong as the rest. The tube was then heated in si, 

 bunsen- burner, and the appearance of the mercury watched 

 from behind a thick plate-glass screen. The liquid mercury 

 attained a full red heat, but nothing could be seen in the 

 part of the tube occupied by the vapour. The temperature 

 was further increased by means of a Herapath blowpipe, and 

 subsequently by placing the tube inside a Fletcher's injector- 

 furnace. At the highest temperatures attained (probably 

 above the melting-point of silver) the vapour in the space 

 above the mercury showed a pale steely-blue tint, which, so 

 far as could be judged, was due to absorption. This tint is 

 probably analogous to the green colour of gold-leaf as seen 

 by transmitted light. The liquid mercury, on the other hand, 



* ' Methods of Glass-blowing.' 



