Conductivity of Metals and their Vapours. 605 



is a conductor. Since the liquid and saturated vapour are 

 indistinguishable above the critical temperature, one or both 

 of these must undergo a remarkable change of electrical 

 properties as that temperature is approached. 



(2) Attempts to predict the critical temperature of mer- 

 cury seem to lead to results altogether inconsistent with one 

 another. 



(3) Attempts to observe the critical phenomena of mer- 

 cury and arsenic in quartz tubes have failed. In both cases 

 experiment proves that the critical temperature lies above a 

 dull yellow heat. 



(4) Up to a full red heat the conductivity of saturated 

 mercury vapour remains of quite a different order of magni- 

 tude from that of the liquid, the latter being 10 million (10 7 ) 

 times as great as the former. But, on the other hand, the 

 conductivity of the saturated vapour is immensely greater 

 than that of the vapour at atmospheric pressure. For 

 the former was found to have a resistance JO 7 times that 

 of the liquid, the latter more than 4 x 10 14 that of the 

 liquid. 



Thus the vapour at atmospheric pressure has a resistance 

 about 4 x 10" times that of the saturated vapour, both at a full 

 red heat. It need scarcely be said that this ratio is of 

 quite a different order from the ratio of the densities of 

 these vapours. 



It seems likely that as the critical temperature is ap- 

 proached the vapour begins to conduct freely, while the 

 liquid changes its electrical character to a much less extent. 



:5) The conductivity of saturated arsenic vapour at a 

 bright red heat is of the same order as that of mercury, and 

 obeys Ohm's law, at all events up to an electromotive intensity 

 of more than 100 volts per cm. 



I do not think it is likely that the critical phenomena 

 of metals will ever be observed. Ii would, however, be 

 worth while to make more elaborate observations on the re- 

 sistance of mercury as the temperature rises. This could 

 perhaps be best done by heating the mercury electrically, the 

 necessary pressure being applied by means of hydraulic 

 testing machinery. I propose to attempt this as soon as I 

 have command of the necessary appliances. 



Terling Tlace, AVitham, Sept. 1902. 



