The Mechanical Action of Radiation. 



5 



Neesen's apparatus consisted of a rectangular case of sheet 

 iron with an aperture in one side closed by a glass plate 

 near to which hung a small and delicately suspended mirror. 

 The radiations were received by the glass plate, through 

 which they passed to fall upon the mirror beyond, and the 

 mirror was compelled to turn in obedience to their influ- 

 ence. {Nature, xiii, 10.) 



By the experiments of these several observers it is well 

 established that very light and mobile bodies are affected 

 quite differently by radiant heat or light according as they 

 are suspended in air or in vacuo. Attraction in air and 

 repulsion in vacuo are the terms employed by Mr. Crookes 

 to describe these effects. These terms are convenient, but 

 unobjectionable only when w^e use them to indicate the 

 direction of the motion and not to describe the nature of 

 the forces acting to produce it. 



Of the nature of these forces views are not yet in accord. 

 Mr. Crookes considers the air-current theory as altogether 

 inconapetent to account either for the attraction or the re- 

 pulsion, but awaits the accumulation of all the facts before 

 attempting to explain any of them. Prof. Devvar regards 

 the heating of the movable disk as the cause of the motion. 

 He is reported as saying that " While the action takes 

 place in air of ordinary density it is probably due to air 

 currents" but, from the report, he seems not to have based 

 this opinion upon any direct experimental proof. Nor 

 does he allow the repulsion in vacuo to be due to any new 

 force of repulsion : he attempts to refer it to the molecular 

 energy of the minute residuum of gas still left in the most 

 perfect attainable vacuum. " What takes place," he says, 

 " is this, the particles are flying in all directions with ve- 

 locity depending upon the temperature. When they im- 

 pinge against the heated disk their velocit}^ is increased. 

 They go off with a greater velocity than those which go 

 off from the colder side and hence there is a recoil of the 



