INSTRUMENTS CONNECTED WITH FLUIDS. 527 



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 discharge 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 perfect that the viscosity and con- 

 ductivity begin sensibly to diminish, even when the stratum of the medium 

 experimented on is very thin. 



But the most remarkable phenomenon hitherto observed in an empty space 

 is that discovered by Mr Crookes. A light body is delicately suspended in an 

 exhausted vessel, and the radiation from the sun, or any other source of light 

 or heat, is allowed to fall on it. The body is apparently repelled and moves 

 away from the side on which the radiation falls. 



This action is the more energetic the greater the perfection of the vacuum. 

 When the pressure amounts to a millimetre or two the repulsion becomes very 

 feeble, and at greater pressures an apparent attraction takes place, which, how- 

 ever, cannot be compared either in regularity or in intensity to the repulsion 

 in a good vacuum. 



From these instances we may see what important scientific discoveries may 

 be looked for in consequence of improvements in the methods of obtaining a 

 vacuum. 



