Repulsion accompanying Radiation, 87 



pith extremities a point of rarefaction can be obtained at which 

 the warm fingers repel and incandescent platinum attracts. 



DuriDg the course of this lecture I have spoken frequently of 

 repulsion by heat, and have used a spirit-lamp as a source of 

 heat to illustrate the facts described. I now wish to show that 

 these results are not confined to the heating rays of the spec- 

 trum,, but that any ray, from the ultra-red to the ultra-violet, will 

 produce repulsion in a vacuum. 



In my own laboratory I have used sunlight, and have experi- 

 mented with a very pure spectrum, taking precautions to avoid 

 any overlapping or diffusion of one part of the spectrum with 

 another. Here I can only use the electric light, and, in order 

 to get results visible at a distance, the spectrum cannot be very 

 long. 



The spectrum is formed with one disulphide-of-carbon prism, 

 and is projected on to the screen by a lens. Immediately be- 

 hind the screen is an exhausted bulb, having a movable index 

 with pith terminals suspended with a cocoon fibre (fig. 3). This 

 is delicate enough to swing over 90° with a touch of the finger, 

 and it will even move under the influence of a ray of moonlight. 

 I first of all arrange the spectrum so that the extreme red would 

 fall on one pith disk were it not for the screen. On removing 

 the screen the index immediately retreats, making nearly half a 

 revolution. 



I now replace the screen, and arrange the spectrum so that 

 the invisible ultra-violet rays are in a position to fall on the pith 

 disk. On removing the screen the index at once behaves as it 

 did under the influence of the red rays, and is driven away 

 twenty or thirty degrees. The action is not so powerful as when 

 the other end of the spectrum is used ; but this may partly, if 

 not wholly, be accounted for by the much greater concentration 

 of energy at the red end of the spectrum, and expansion at the 

 violet end, when using glass or disulphide-of-carbon prisms. 



I now, without disturbing the position of the spectrum, inter- 

 pose in the path of the rays a cell containing a solution of iodine 

 in disulphide of carbon, which is opaque to the luminous and 

 ultra-violet rays, but transparent to the invisible heat-rays. Not 

 a trace of repulsion is produced. The iodine solution is now re- 

 moved and the ultra-violet rays again fall on the pith, producing 

 strong repulsion. A thick screen of clear alum cut from one of 

 Mr. Spencers gigantic crystals is now interposed ; but no effect 

 whatever is produced by it, the ultra-violet rays acting with un- 

 abated energy. As alum cuts off all the dark heat- rays, this 

 experiment and the one before it prove the sufficient purity 

 of my spectrum. 



The spectrum is again turned until the dark ultra-red heating 



