caused by Evaporation and Condensation. 151 



be in opposition to the lifting of the ascending current, and it will 

 depend on the shape and thickness of the ball whether it will rise 

 or fall when in an ascending current of heated gas. 



The reason why Mr. Crookes did not obtain the same results 

 with a less perfect vacuum was because he had then too large a 

 proportion of air, or non-condensing gas, mixed with the vapour, 

 which also was not in a state of saturation. In his experiments 

 the condensable vapour was that of mercury, or something which 

 required a still higher temperature, and it was necessary that the 

 vacuum should be very perfect for such vapour to be any thing 

 like pure and in a saturated condition. As soon, however, as this 

 state of perfection was reached, then the effects were more appa- 

 rent than in the corresponding case of water. This agrees well 

 with the explanation; for, as previously shown, the effect of mercury 

 would, for the same quantity of heat, be three times as great as 

 that of water ; and, besides this, the perfect state of the vacuum 

 would allow the pith (or whatever the ball might be) to move much 

 more freely than when in the vapour of water at a considerable 

 tension. 



Of course this reasoning is not confined to mercury and water ; 

 any gas which is condensed or absorbed by the balls when cold 

 in greater quantities than when warm would give the same re- 

 sults ; 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 evi- 

 dent 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 temperature, 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 



