* 
Physics and Chemistry. 149 
vacuum, is not questioned, but this is regarded to be due not to a 
sudden increase in resistance of the vacuum itselfbut to the 
the surrounding medium. Representing by 7, the specific resist- 
ance in a ¢ f gas of unit length, and by r the resistance to 
the passage from the electrode to the medium, then the total resist- 
ance for a tube of length 7 will be r+7, fessor Edlund 
and so on), after which it must increase. As 1,, for theoret- 
ical reasons, diminishes as the rarefaction is increased, this fact 
Just stated can be explained only by assuming that 7 ine 
Maxwett, M.A., edited by Wiiiiam Garner, M.A. 268 pp. 
with 6 plates. Oxford, 1881 (Clarendon Press).—The larger por- 
