1874.] caused by Evaporation and Condensation. 405 



planation of the experiments previously described. When the radiated 

 heat from the lamp falls on the pith, its temperature will rise, and any 

 moisture on it will begin to evaporate and to drive the pith from the 

 lamp. The evaporation will be greatest on that ball which is nearest to 

 the lamp ; therefore this ball will be driven away until the force on the 

 other becomes equal, after which the balls will come to rest, unless 

 momentum carries them further. On the other hand, when a piece of 

 ice is brought near, the temperature of the pith will be reduced, and it 

 will condense the vapour and be drawn towards the ice. 



It seems to me that the same explanation may be given of Mr. Crookes's 

 experiments ; for, although my experiments were made on water and at 

 comparatively high pressures, they were in reality undertaken to verify 

 the explanation as I have given it. I used water in the hope of finding 

 (as I have found) that, in a condensable vapour, the results could be 

 obtained with a greater density of vapour (that is to say, with a much lesa 

 perfect vacuum), the effect being a consequence of the saturated condition 

 of the vapour rather than of the perfection of the vacuum. 



Mr. Crookes only obtained his results when his vacuum was nearly as 

 perfect as the Sprengel pump would make it. Up to this point he had 

 nothing but the inverse effects, viz. attraction with heat and repulsion 

 with cold. About the cause of these he seems to be doubtful ; but I 

 venture to think that they may be entirely explained by the expansion of 

 the surrounding gas or vapour, and the cons3quent convection-currents. 

 It must be remembered that whenever the air about a ball is expanded, 

 and thus rendered lighter by heat, it will exercise less supporting or 

 floating power on the ball, which will therefore tend to sink. This ten- 

 dency will 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 tempera l/ure, 

 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 ot! perfection was reached, then, the effects were 

 more apparent 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 



