MOLECULAR PHYSICS. 91 



MM. Kundt and Warburg have got rid of an additional quantity 

 of water-substance by heating the vessel to as high a temperature 

 as the glass will bear while the pump was kept in action. 



A method which has been long in use for getting a good 

 vacuum is to place in the vessel a stick of fused potash, and to fill 

 it with carbonic acid, and, after exhausting as much as possible, to 

 seal up the vessel. The potash is then heated, and when it has 

 again become cold, most of the remaining carbonic acid has com- 

 bined with the potash. 



Another method, employed by Professor Dewar, is to place in a 

 compartment of the vessel" a piece of freshly heated cocoa-nut 

 charcoal, and to heat it strongly during the last stages of the 

 exhaustion by the mercury-pump. The vessel is then sealed up, 

 and as the charcoal cools it absorbs a very large proportion of the 

 gases remaining in the vessel. 



The interior of the vessel, after exhaustion, is found to be pos- 

 sessed of very remarkable properties. 



One of these properties furnishes a convenient test of the com- 

 pleteness of the exhaustion. The vessel is provided with two 

 metallic electrodes, the ends of which within the vessel are within 

 a quarter of an inch of each other. When the vessel contains air 

 at the ordinary pressure a considerable electromotive force is 

 required to produce an electric discharge across this interval. As 

 the exhaustion proceeds the resistance to the discharge diminishes 

 till the pressure is reduced to that of about a millimetre of mercury. 

 When, however, the exhaustion is made very perfect the dis- 

 charge cannot be made to take place between the electrodes 

 within the vessel, and the spark actually passes through several 

 inches of air outside the vessel before it will leap the small interval 

 in the empty vessel. A vacuum, therefore, is a stronger insulator 

 of electricity than any other medium. 



MM. Kundt and Warburg have experimented on the viscosity 

 of the air remaining after exhaustion, and on its conductivity for 

 heat. They find that it is only when the exhaustion is very per- 



