292 Prof. J. H. Poynting on Osmotic Pressure. 



observe any double refraction after the shearing motion ceased. 

 Kundt (Ann. Pliys. CJiem. xiii. 1881) made a series of expe- 

 riments and found double refraction in many sheared liquids, 

 notably in olive-oil, but never in a pure liquid with a definite 

 chemical constitution. The more complex the molecules 

 apparently the less is X, and the greater is the shear strain 

 still remaining at any instant of the motion. But in liquids 

 such as water or glycerine, the decay is so rapid that no 

 optically appreciable amount remains. 



Still it is very possible that olive-oil is only an extreme 

 case, and that water and other apparently inactive liquids 

 would show the effect if we could sufficiently increase the 

 shear, and I think Kundf s results may be claimed as sup- 

 porting the hypothesis. Possibly, too, the observation of 

 Quincke, that double refraction is observed in a liquid close 

 to a very hot wire, gives further support. The unequal 

 heating may perhaps be regarded as producing shear strains 

 in the solid structure which are renewed by the supply of 

 heat as fast as they break down. 



In the case of breaking down of structure near the surface 

 of a liquid the moving molecules may succeed in escaping 

 altogether, and may fly off as gas molecules if they are 

 directed upwards and have enough energy. Of course there 

 may be many molecules able to move about and yet not able 

 to evaporate ; for though they may be able to travel when in 

 the body of the liquid, they may not have energy enough to 

 get clear away from their neighbours when these are all on 

 one side and all pull in one direction as they do at the surface. 

 In the case of practically non-evaporating liquids, such as 

 mercury at ordinary temperatures, we must suppose that only 

 a very minute fraction are thus able to do the work needed 

 to overcome the large cohesion of their neighbours. 



It will be convenient to use the term " mobility " to de- 

 scribe the number of "free" or " mobilised " molecules 

 crossing a square centimetre per second in a liquid, where by 

 " free " or " mobilised " we mean those which are changing 

 their surroundings and forming new connexions. Evidently 

 we may extend the term to a gas, remembering that then all 

 the molecules are mobilised, and that the mobility is propor- 

 tional to the pressure. 



When a square centimetre is taken on the surface of a 

 liquid, the mobility upwards is the rate of evaporation, and 

 the mobility of the vapour downwards is the rate of conden- 

 sation. When the two mobilities are equal the pressure of 

 the vapour is the vapour-tension. 



The mobility in the body of the liquid is probably far 



