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



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 sur- 

 face is attended with a force tending to drive the surface back, and 

 condensation 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 lire with 

 a perceptible velocity ; and therefore 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 for- 

 wards. And, in a similar way, whenever condensation goes on at 

 a surface it must diminish 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 be insensible, except when the heat on the 

 surface was sufficiently intense to drive the vapour off with con- 

 siderable velocity. This, indeed, might be the case if vapour had 

 no particles and was, what it appears to be, a homogeneous elastic 

 medium, and if, in changing from liquid into gas, the expansion 

 took place gradually, so that the only velocity acquired by the vapour 

 was that necessary to allow its replacing that which it forces 

 before it and giving place to that which follows. 



But, although it appears to have escaped notice so far, it follows, 

 as a direct consequence of the kinetic theory of gases, that, when- 

 ever evaporation takes place from the surface of a solid body or a 

 liquid, it must be attended with a reactionary force equivalent to 

 an increase of pressure on the surface, which force is quite in- 

 dependent of the perceptible motion of the vapour. Also, conden- 

 sation must be attended with a force equivalent to a diminution of 

 the gaseous pressure over the condensing surface, and likewise 

 independent of the visible motion of the vapour. This may be 

 shown to be the case as follows : — 



According to the kinetic theory, the molecules which constitute 

 the gas are in rapid motion, and the pressure which the gas exerts 

 against the bounding surfaces is due to the successive impulses of 

 these molecules, whose course directs them against the surface, from 

 which they rebound with unimpaired velocity. According to this 

 theory, therefore, whenever a molecule of liquid leaves the surface 

 henceforth to become a molecule of gas, it must leave it with a 

 velocity equal to that with which the other particles of gas re- 

 bound ; that is to say, instead of being just detached and quietly 



