1881.] 



Radiant Energy into Sonorous Vibrations. 



509 



The best effect was obtained when AB was a thin ebonite wire about 

 "5 millim. in diameter, and 6 centims. in length. 



It was evident from these experiments that the sonorous effects of 

 hard disks could not be explained by the change of volume due to the 

 impact of heat rays, and their absorption by the mass of the disk.* 



Is the action then due to molecular pressure similar to that which 

 produces the rotation of the radiometer ? 



It is quite true that the radiometer effect is one visible only in very 

 high exhaustions, but Mr. Crookes (" Phil. Trans.," vol. 169, § 220) 

 detected "the existence of molecular pressure when radiation falls on 

 a black surface in air of normal density." 



Whenever radiant energy falls on an absorbent surface in air, such 

 as a disk of blackened wood, its wave-length is degraded or lowered, 

 and it is converted in thermometric heat. The molecules of air 

 striking this warmed surface acquire heat, and move away from it 

 with increased velocity, and as action and reaction are always alike in 

 moving away, they give the body a " kick." Since there is no such 

 action on the other side of the disk, there is a difference of pressure 

 between the two sides, which gives it a tendency to move away from 

 the source of energy. The effect is very much smaller in air at 

 •ordinary pressures than in air at a very low vacuum, because in the 

 former case, the mean free path of the molecules is very small, and 

 the rebounding molecules help to keep back the more slowly ap- 

 proaching molecules. Nevertheless, molecular pressure is experienced, 

 and if of sufficient magnitude and rapidity, it ought to produce 

 sonorous vibrations. It seemed probable that the element of time 

 does not enter here so prominently as in the previous case, for the 

 radiometer effect is a mere surface action of the disk, and not one 

 affecting its mass. Hence it was hoped that the retarding effects 

 would be eliminated. If the sonorous action, therefore, be due to a 

 radiometer action, a difference of effect would be observed if the side 



* Tnis conclusion was confirmed by subsequent experiments, notably by Experi- 

 ments 7 and 41. 



