406 Prof. O. Eeynolds on Surface-Forces [June 18," 



quantities than when warm would give the same results ; and, as this 

 property appears to belong to all gases, it is only a question of bringing 

 the vacuum to the right degree of tension. 



There was one fact connected with Mr. Crookes's experiments which, 

 independently of the previous considerations, led me to the conclusion 

 that the result was due to the heating of the pith, and was not a direct 

 result of the radiated heat. 



In one of the experiments exhibited at the Soiree of the Royal Society, 

 a candle was placed close to a flask containing a bar of pith suspended 

 from the middle : at first, the only thing to notice was that the pith was 

 oscillating considerably under the action of the candle ; each end of the 

 bar alternately approached and receded, showing that the candle exercised 

 an influence similar to that which might have been exercised by the torsion 

 of the thread had this been stiff. After a few minutes' observation, 

 however, it became evident that the oscillations, instead of gradually 

 diminishing, as one naturally expected them to do, continued ; and, more 

 than this, they actually increased, until one end of the bar passed the light, 

 after which it seemed quieter for a little, though the oscillations again 

 increased until it again passed the light. As a great many people and 

 lights were moving about, it seemed possible that this might be due to 

 external disturbance, and so its full importance did not strike me. 

 Afterwards, however, I saw that it was only to be explained on the 

 ground of the force being connected with the temperature of the pith. 

 During part of its swing one end of the pith must be increasing in tem- 

 perature, and during the other part it must be cooling. And it is easily 

 seen that the ends will not be hottest when nearest the light, or coldest when 

 furthest away ; they will acquire heat for some time after they have begun 

 to recede, and lose it after they have begun to approach. There will, in 

 fact, be a certain lagging in the effect of the heat on the pith, like that 

 which is apparent in the action of the sun on a comet, which causes the 

 comet to be grandest after it has passed its perihelion. From this cause 

 it is easy to see that the mean temperature of the ends will be greater 

 during the time they are retiring than while approaching, and hence the 

 driving force on that end which is leaving will, on the whole, more than 

 balance the retarding force on that which is approaching ; and the result 

 will be an acceleration, so that the bar will swing further each time until 

 it passes the candle, after which the hot side of the bar will be opposite 

 to the light, and will for a time tend to counteract its effect, so that the 

 bar will for a time be quieter. This fact is independent evidence as to 

 the nature of the force ; and although it does not show it to be evapora- 

 tion, it shows that it is a force depending on the temperature of the pith, 

 and that it is not a direct result of radiation from the candle. 



Since writing the above paper, it has occurred to me that, according to 

 the kinetic theory, a somewhat similar effect to that of evaporation must 

 result whenever heat is communicated from a hot surface to gas. 



