92 SCIENTIFIC APPARATUS. 



feet that the viscosity and conductivity 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, however, cannot be com- 

 pared either in regularity or in intensity to the repulsion in a good 

 vacuum. 



From these instances we may see what important scientific dis- 

 coveries may be looked for in consequence of improvements in 

 the methods of obtaining a vacuum. 



J. CLERK MAXWELL. 



