Conductivity of Metals and their Vapours. 597 



Jf the electrodes were each 10 x 1 cms. in dimensions, and 



1 cm. apart, the specific resistance would then be no less than 



157 



= 8 x 10 u ohms, rouo-hlw On the other hand, the 



2xl0" 10 —,^-e 



specific resistance of liquid mercury at ordinary temperatures 

 is about 10"' 4 ohms. At a yellow heat, it would not at the 

 most be more than 5 times this amount, according to the 

 experiments which have been made on the variation of its 

 resistance with temperature ; this would make the liquid 

 resistance 2 X 10~ 3 ohms. But in all probability it is not more 

 than half as much. Thus, at a yellow heat, so far as can be 

 judged from existing data, the resistance of the vapour, at 

 atmospheric pressure, should be 



2 x 10~ 3 

 that of the liquid. 



This stupendous difference of properties is very remarkable. 

 And the question presents itself, what changes do the resist- 

 ance of the liquid and of the vapour respectively undergo. 

 as the critical temperature and pressure are approached? 

 It must be supposed that, since above that temperature 

 the liquid and the saturated vapour are indistinguishable, 

 they have the same electrical resistance, whether that resist- 

 ance be high or low. In what manner does this wonderful 

 change of electrical properties set in ? Is it gradual or is it 

 abrupt, like the change in the magnetic permeability of iron 

 at high temperatures ? 



I have not succeeded in going far towards an answer to 

 this question, but have thought it desirable to record such 

 small progress as I have been able to make. 



§ 2. On the Probable Values of the Critical Temperatures 

 of Metals. 



There are various methods by which some estimate of the 

 critical temperatures of ordinary liquids may be made in the 

 absence of direct observations. These, however, lead to 

 hopelessly discrepant results when it is attempted to apply 

 them to mercury. One of these methods depends on the 

 temperature coefficient of the surface-tension of the liquid. 

 Since the surface-tension of a liquid is a linear function of 

 the temperature*, it is easy to find by extrapolation the 

 temperature at which the surface-tension would vanish. 



* There is reason to think that this 3urface-tension should be multiplied 

 by the (specific volume)^ for the linear relation to hold strictly. But this 

 hardly affects the result in the case of mercury. 



