THE MECHANICAL ACTION OF LIGHT. 



259 



What, then, is the cause of the contradictory action in these two 

 bulbs attraction in one, and repulsion in the other? It can be ex- 

 plained in a few words. Attraction takes place when air is present, 

 and repulsion when air i^febsent. 



Neutrality, or no movement, is produced when the vacuum is insuf- 

 ficient. A minute trace of air in the 

 apparatus interferes most materially 

 with the repulsion, and for a long 

 time I was unaware of the powerful 

 action produced by radiation in a 

 " perfect " vacuum. 



It is not at first sight obvious 

 how ice or a cold body can produce 

 the opposite effect to heat. The law 

 of exchanges, however, explains this 



i 



Fig. 1. 



Fig. 2. 



perfectly. The pith-bar and* the whole of the surrounding bodies are 

 incessantly exchanging heat-rays ; and under ordinary circumstances 

 the income and expenditure of heat are in equilibrium. Let me draw 

 your attention to the diagram (Fig. 2), illustrating what takes -place 

 when I bring a piece of ice "near the apparatus. The centre circle 

 represents my piece of pith ; the arrows show the influx and efflux of 

 heat. A piece of ice brought near cuts off the influx of heat from one 

 side, and therefore allows an excess of heat to fall on the pith from 

 the opposite side. Attraction by a cold body is therefore seen to be 

 only repulsion by the radiation from the opposite side of the room. 



The later developments of this , research have demanded the 

 utmost refinement of apparatus. Everything has to be conducted in 

 glass vessels, and these must be blown together till they make one 

 piece, for none but fused joints are admissible. In an investigation 

 depending for its successful prosecution on manipulative dexterity, I 

 have been fortunate in having the assistance of my friend Mr. Charles 

 Gimingham. All the apparatus you see before you are the fruits of 

 bis skillful manipulation, and I now want to draw your attention to 



