1886.] On Thickness and Surface-tension in Liquid Films. 445 



This conclusion is based upon a method of experiment by which a 

 change of \ per cent, in the value of the tension must have been 

 detected, had it existed, and upon fifteen independent comparisons of 

 the tensions of black and coloured films. 



The whole of the observations were carried out under the conditions 

 which the previous researches of the authors have shown to be 

 necessary to maintain the composition and the temperature of the 

 films unchanged. 



The authors next discuss the bearing of their observations upon 

 the question of the magnitude of the so-called "radius of molecular 

 attraction." They point out that if the mere eqnality in the surface- 

 tensions of thick and thin films is to be considered conclusive, they 

 have accumnlated much stronger evidence for the statement that the 

 radius of molecular attraction is less than half the thickness of a 

 black film, i.e., <6 x 10 -6 mm. than Platean produced for the assertion 

 that 59 X 10 -6 mm. is a superior limit to its magnitude. They are, 

 however, unwilling to draw thi3 conclusion from their experiments 

 until an explanation is forthcoming, in harmony with it, of the 

 apparent discontinuity in the thickness of the film which always 

 (except under very special circumstances) occurs at the edge of the 

 black. 



They are themselves inclined to look upon the sharp edge of the 

 black as evidence of a change in surface-tension due to the tenuity of 

 the film, and to regard the result of their experiments as fixing a 

 superior limit (0*5 per cent.) to the difference of the tension of the 

 black and coloured parts. 



As no explanation of the discontinuity at the edge of the black has 

 (as far as the authors are aware) ever been put forward, they 

 conclude by a suggestion which, though no doubt of a speculative 

 character, may serve to draw attention to a subject which is they 

 believe of considerable interest. 



They show that the main facts to be accounted for, viz., the dis- 

 continuity, the uniform thickness of the black, the wide variations in 

 the thickness of the part of the coloured film which is in contact 

 with the black, and the equality in the surface- tensions of the black 

 and coloured films, could be explained if it were supposed that the 

 surface-tension has a critical value when the thickness is somewhat 

 greater than 12 x 10~ 6 mm. 



The possibility of the existence of such a critical value has been 

 pointed out by Maxwell.* It would be explained by the assumption 

 frequently made in discussions on the nature of molecular forces, 

 that as the distance between two molecules diminishes, the mutual 

 force between them is alternately attractive and repulsive. 



* " Encycl. Brit.," art. "Capillarity." 



