150 Royal Society : — Prof. 0. Reynolds on Surface-forces 

 therefore 



Thus we have an expression for the force in terms of the quan- 

 tity of water evaporated and the ratio of the pressure to the den- 

 sity of the vapour; and if the heat necessary to evaporate the 

 liquid (the latent heat) is known, we can find the force which 

 would result from a given expenditure of heat. 



Applying these results to steam, we find that, at a temperature 

 of 60°, the evaporation of 1 lb. of water from a surface would be 

 sufficient to maintain a force of 65 lbs. for one second. 



It is also important to notice that this force will be proportional 

 to the square root of the absolute temperature, and, consequently, 

 will be approximately constant between temperatures of 32° and 

 212°. 



If we take mercury instead of water, we find that the force is 

 only 6 lbs. instead of 6o lbs. ; but the latent heat of mercury is only 

 ■gL that of water, so that the same expenditure of heat would main- 

 tain nearly three times as great a force. 



It seems, therefore, that in this way we can give a satisfactory 

 explanation 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 tem- 

 perature of the pith will be reduced, and it will condense the va- 

 pour 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 conden- 

 sable vapour, the results could be obtained with a greater density 

 of vapour (that is to say, with a much less 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 inake 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 consequent 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 tendency will 



