10 



The Mechanical Action of Radiation. 



Whether this repulsion in air is at all related to the re- 

 pulsion in vacuo, I am not prepared to consider clear. 

 The effect of exhaustion, on the relative strength of these 

 two forces, is, however, an interesting point in this con 

 nection. In a good vacuum the power of convection must 

 be vastly reduced and as the vacuum approaches perfec- 

 tion, this power must approach annihilation. On the 

 other hand nothing short of an absolute vacuum can forbid 

 the molecular transmission of energy. It seems highly 

 probable that at a certain high degree of exhaustion this 

 molecular energy would be the more powerful of th6 two, 

 and hence that repulsion would be the normal instead of 

 the exceptional indication of the instrument. 



