2o8 PHYSICAL SCIENCE 



of area. By continual extension it would be 

 possible to expend an unlimited quantity of work, 

 as long as no change in the nature of the film 

 took place under the influence of the progressive 

 expansion and consequent attenuation of the film. 

 The point at which it is natural to expect that 

 some change would occur is that moment when 

 the two sides of the film have been brought so 

 near to each other, by the process of continual 

 thinning, that the outside faces confining the film 

 come within range of each other's molecular 

 forces. But, however far the film be extended, 

 it is evident that, as long as it remains a film, 

 less work must be used than could otherwise be 

 expended in evaporating the film and converting 

 its substance into steam, since by this means its 

 molecules would be separated completely from 

 each other's sphere of influence. The value of 

 this latter amount of work is known from other 

 experiments, and is measured by the latent heat 

 of evaporation of the substance of the film, which 

 is composed almost entirely of water. It is 

 possible, therefore, to calculate for the film a 

 hypothetical thickness, certainly less than the 

 critical thickness at which it would begin to 

 show new properties owing to the approach of 

 the opposite faces within molecular distances. 

 Numerical results show that this limiting thick- 

 ness may be put at about io~^ of a centimetre. 

 There are thus not more than ten million molecules 

 in a row in a length of a millimetre, and two 

 hundred million in the space of an inch. The 

 numbers in the corresponding volumes will be 

 found by cubing these values ; a cubic centimetre 

 of water contains not more than lo'"^ molecules. 



