402 Prof. O. Reynolds on Surface-Forces [June 18, 



while the other ball would become dry (this would be seen to be the 

 case, and was also shown, by the tipping of the balance, that ball against 

 the ice gradually getting lower). It was then found, when the ice was 

 removed, that the dry ball was insensible to the heat, or nearly so, while 

 that ball which had been opposite to the ice was more than ordinarily 

 sensitive. 



If the flask were dry and the tension of the vapour reduced with the 

 pump until the gauge showed | of an inch, then, although purely steam, 

 the vapour was not in a saturated condition, and the pith-balls which 

 were dry were no longer sensitive to the lamp, although they would still 

 approach the ice. 



From these last two facts it appears as though a certain amount of 

 moisture on the balls was necessary to render them sensitive to the heat. 



In order that these results might be obtained, it was necessary that 

 the vapour should be free from air. If a small quantity of air was 

 present, although not enough to appear in the gauge, the effects rapidly 

 diminished, particularly that of the ice, until the convection-currents had 

 it all their own way. This agrees with the fact that the presence of a 

 small quantity of air in steam greatly retards condensation and even 

 evaporation. 



With a dry flask and an air-vacuum, neither the lamp nor the ice 

 produced their effects ; the convection-currents reigned supreme even 

 when the gauge was as low as J inch. Under these circumstances the 

 lamp generally attracted the balls and the ice repelled them, i.e. the 

 currents carried them towards the lamp and from the ice ; but, by placing 

 the lamp or ice very low, the reverse effects could be obtained, which 

 goes to prove that they were the effects of the currents of air. 



These experiments appear to show that evaporation from a surface is 

 attended with a force tending to drive the surface back, and condensa- 

 tion with a force tending to draw the surface forward. These effects 

 admit of explanation, although not quite as simply as may at first sight 

 appear. 



.It seems easy to conceive that when vapour is driven off from a body 

 there must be a certain reaction or recoil on the part of the body ; Hero's 

 engine acts on this principle. If a sheet of damp paper be held before 

 the fire, from that side which is opposite to the fire a stream of vapour 

 will be drawn off towards the fire with a perceptible velocity ; and there- 

 fore we can readily conceive that there must be a corresponding reaction, 

 and that the paper will be forced back with a force equal to that which 

 urges the vapour forwards. And, in a similar way, whenever condensa- 

 tion goes on at a surface it must dimmish the pressure at the surface, 

 and thus draw the surface forwards. 



It is not, however, wholly, or even chiefly, such visible motions as these 

 that afford an explanation of the phenomena just described. If the only 

 forces were those which result from the perceptible motion, they would 



