6 



The 31echanical Action of Radiation. 



di^k." And this recoil he thinks competent to put the 

 disk in motion even in his excellent vacuum " where we 

 know that the exhaustion has reduced the density to 

 iinnS-inny original." 



Prof. Osborne Reynolds claims that whenever a surf\ice 

 imparts heat to a gas, there must be a reaction of the gas 

 against the surface. A surface free to move would be 

 propelled by this reaction. Moreover in a vacuum this 

 motion would encoujiter less resistance and hence would 

 he more conspicuous, as if it were due to a stronger force. 

 Repulsion in vacuo, according to this view, is the etfect of 

 this gaseous reaction. {Nature^ vol. xii, 405.) 



If others have engaged in the investigation of this inter- 

 esting subject, their work has, down to the present time, 

 escaped my attention. 



In July, 1875, I read a paper before the Poughkeepsie 

 Society of Natural Science giving a synopsis of several series 

 of experiments some of which were made before the pub- 

 lication of Mr. Crookes' remarkable discovery, but many 

 of them afterward, with a view to determine the nature of 

 the force as fj.iras the so called attraction in air is concerned 

 (Jour. Frank Inst., vol. lxx, 134). The points then made 

 are these : 1st. The motion of the needle having proved to 

 be, under a great variety of conditions, exactly what air- 

 currents are competent to produce, we may justly infer 

 that it is due to convection. 2d. If this motion shall cease 

 to occur when the air is removed (and Mr. Crookes has 

 proved that it does), the evidence in favor of convection is 

 complete. 



Whatever may be the nature of the repulsion in vacuo, 

 convection will, doubtless, in the end, be admitted to be 

 the cause of the motion in air. The two motions are, I 

 am persuaded, altogether distinct phenomena manifesta- 

 tions of two antagonistic forces, the repulsive being the 

 more delicate and able to produce its effect only when the 



