THE SIZE OF ATOMS. 171 



saying that the film cannot keep up its tensile 

 strength to i/ioo,ooo,oooth of a centimetre, and 

 that is, that the work which would be required 

 to stretch the film a little more than that would 

 be enough to drive it into vapour. 



The theory of capillary attraction shows that 

 when a bubble a soap-bubble, for instance is 

 blown larger and larger, work is done by the 

 stretching of the film which resists extension as if 

 it were an elastic membrane with a constant 

 contractile force. This contractile force is to be 

 reckoned as a certain number of units of force 

 per unit of breadth. Observation of the ascent of 

 water in capillary tubes shows that the contractile 

 force of a thin film of water is about 16 milli- 

 grammes weight per millimetre of breadth. Hence 

 the work done in stretching a water film to any 

 degree of thinness, reckoned in millimetre-milli- 

 grammes, is equal to sixteen times the number 

 of square millimetres by which the area is aug- 

 mented, provided the film is not made so thin 

 that there is any sensible diminution of its con- 

 tractile force. In an article " On the Thermal 



