[ 250 J 

 XXXI. Intelligence and Miscellaneous Articles. 



ON THE ATTRACTION AND REPULSION EXERTED BY THE LUMI- 

 NOUS AND THE CALORIFIC RAYS : By DR. F. NEESEN*. AND 

 ON CROOKES'S RADIOMETER : BY M. POGGENDORFFf. 



"V/TE. CROOKES'S remarkable experiments relating to the pheno- 

 •*■*■*• mena of attraction and repulsion which, he says, result from 

 calorific and luminous radiations, could not fail to fix the attention 

 of physicists ; yet the explanation which he gives of them must of 

 necessity call forth numerous objections. It is, in fact, difficult to 

 attribute to radiation the power to produce mechanical effects by 

 acting through a vacuum — that is, with no other intermedium than 

 the aether or extremely rarefied particles. 



On the other hand, the high degree of rarefaction of the medium 

 in which Mr. Crookes effected his experiments seems at first view 

 to exclude the possibility of explaining the facts by merely the 

 action of currents resulting from the inequalities of temperature of 

 that medium, and especially the continuous rotation which he 

 obtains in his radiometer. 



Nevertheless the more recent researches of Dr. Neesen seem to 

 prove that it is indeed currents of this nature that play the principal 

 part in the phenomena in question. 



M. Neesen used an apparatus which, it is true, does not permit 

 the realization of so perfect a vacuum as that in which Mr. 

 Crookes's pendulums or radiometer move, but which has the ad- 

 vantage of permitting the light or heat to be made to act in a 

 perfectly determinate direction, and upon an equally determinate 

 part of the suspended body. 



This apparatus consists of a tin box, one of the sides of which 

 has an opening fitted with a parallel-faced glass plate, giving pas- 

 sage to the calorific or luminous rays. The pendulum is fastened 

 to the top of the box by means of a movable frame, so that it can 

 be brought near to the glass plate or to the opposite side. It con- 

 sists of a cocoon-thread to which is attached a piece of pasteboard 

 or of wood covered with paper, carrying in its centre a small plane 

 mirror, the deflections of which, observed at a distance with the 

 cathetometer, measure the torsion of the thread. Finally, the 

 light and heat are derived from a petroleum-lamp, the rays from 

 which can be projected at will upon one of the sides of the pen- 

 dulum, sometimes directly, sometimes concentrated by means of 

 a lens. 



Keeping at first the air contained in the box at the atmospheric 

 pressure, the author satisfied himself that the radiation from the 

 lamp really determines in it currents of transport. This appears 

 clearly from the fact that the action of the lamp is different ac- 

 cording as the pendulum is nearer to or further from the glass. 

 By varying this distance it is ascertained that the deflection shown 

 by the mirror is almost none when the pendulum is close to the 



* Pogg. Ann, September 1875. t Ibid. November 1875. 



