252 Intelligence and Miscellaneous Articles. 



faction, in which, perhaps, it would not be legitimate to admit the 

 production of currents possessing sufficient energy to carry with 

 them the relatively considerable mass of the pendulum. 



It is, besides, very possible that these phenomena are actually 

 due to currents as long as the rarefaction of the air contained in the 

 apparatus does not exceed a certain degree; but it may also very 

 well be that a totally different cause intervenes when the exhaustion 

 is more complete. That cause, still obscure, may perhaps be the 

 one suggested at first by Mr. Eeynolds very shortly after Mr. 

 Crookes's publication*. 



Mr. Eeynolds bases his interpretation on a consequence of the 

 mechanical theory of gases. He remarks that the heating of 

 gaseous particles which strike a surface hotter than themselves, 

 and consequently the augmentation of their velocity, must be fol- 

 lowed by a reaction in a direction opposite to that of the rays 

 which act upon that surface, since the particles rebound with a 

 greater velocity than that which they possessed before the impact. 

 As, on the other hand, thermic radiation as well as the mobility of 

 the particles must augment with the degree of rarefaction of the 

 gas, this effect of reaction, he thinks, may still possess sufficient 

 energy in the most highly rarefied media, where mere currents 

 would have become too feeble. 



Nevertheless M. Poggendorff, who has also occupied himself 

 with this delicate subject, ranges himself rather on the side of the 

 interpretation given by Dr. Neesen. It is true he does not regard 

 the phenomenon as completely elucidated; but he is convinced 

 that the motions, whether of the pendulum or of Crookes's radio- 

 meter, would certainly not take place in a perfect vacuum. 



Using a radiometer constructed by Dr. Geissler, of Bonn, on 

 the model of those employed by Mr. Crookes, M. Poggendorff has 

 proved the perfect accuracy of the facts observed by the latter ; he 

 has even seen the vane of the radiometer put in motion merely 

 by the influence of daylight from a sky covered with clouds. On 

 the other hand, he has proved that the apparatus absolutely ceases 

 to turn when the luminous rays which reach it have passed through 

 a thickness of 20 millims. of water. 



He notices also another fact, not less important. It is known 

 that, in Mr. Crookes's experiments, the rotation of the vane does 

 not take place when it is acted on by the obscure thermic radiation 

 resulting from direct heating of the glass balloon which contains 

 it. Now M. Poggendorff has obtained all the usual rotation effects 

 by projecting upon his radiometer the rays from a wax taper after 

 they have traversed a plate of black glass which is made use of in 

 polarization-experiments, and which owes its opacity to the pre- 

 sence of a certain quantity of sulphur. Mr. Crookes, as well as 

 Messrs. Tait and Dewar, had already proved that it is the same 

 with the obscure radiation emanating from sulphiodide of carbon. 



In brief, the phenomena discovered by Mr. Crookes are not yet 

 completely explained ; but it is probable that they are in great 



* See Reynolds, " On the Forces caused by Evaporation from and Con- 

 densation at a Surface/' Proc. Roy. Soc. June 1874, p. 406. 



