522 Dr. Parks on the Thickness of the Liquid Film 



nucleus, though as the exhaustion o£ the chamher proceeds 

 the average size of the water particles increases by ordinary 

 condensation at temperatures below the dew-point. If we 



Fi-. 2. 

































































-r 



=J 10 



X 



5 













s 





■y 









> 













g 5 













(D 

























30 



Thickness of film X 10 s . 



subtract the average diameter of the nuclei from the average 

 diameter of the drops, and halve the remainder, this will give 

 the thickness of the film of water. 



In the following table the first two columns have been 

 taken from the original paper of Dr. Barus, and the last 

 column has been inserted by the present writer. The results 

 for the first six exhaustions' only have been selected. 



Dr. Bar us remarks that ""the use of Kelvin's vapour- 

 tension equation breaks down quantitatively for the present 

 purposes in practice." The reason for this will be clear 

 when it is remembered that Kelvin's vapour-tension equation 

 is only intended to apply to a condition oi equilibrium 

 existing between a liquid and its vapour ; but in condensation 

 upon solid surfaces another element must be taken account 

 of, viz., a force of the nature of an attraction between the 

 solid and the liquid or vapour, which causes a pressure, 



