SURFACES OF DISCONTINUITY 669 



chanical shock, being much less resistant to this than the 

 thicker, colored films. Nevertheless, with extraordinary pre^ 

 cautions soap films have been kept "alive" for many days, and 

 in one case certainly for a year. For further information on the 

 preparation of the solutions and on the experimental technique, 

 the reader can consult Lawrence's book already mentioned. 



In a vertical film the black stage appears at the top and 

 gradually spreads downwards, the boundary between it and the 

 thicker film immediately below being quite a sharp horizontal 

 line. In the lower part of the film illumination by mono- 

 chromatic light shows, by the appearance of horizontal bands 

 of color across the film, that stages of different thickness succeed 

 one another, the whole mass draining all the time and the 

 banded appearance going through characteristic changes accord- 

 ingly. In a horizontal film the black appears as a small circular 

 disc. The sharp boundary between the black and the adjacent 

 part indicates a change in thickness with a very steep gradient, 

 involving changes occasionally as much as several hundred 

 to one between black and adjacent parts, and never less than 

 ten to one. As stated on p. 662 of this volume, it used to be 

 believed that the appearance of black necessarily led to early 

 rupture of the film, but this is not a fact provided shock and 

 contamination are avoided. The thinning of a horizontal film 

 in this way is of course not due to gravity; actually the Gibbs 

 ring formed where the film meets the solid boundary to which 

 it is attached is responsible for this draining. 



We have referred briefly to the normal thinning of a film, 

 under, of course, careful conditions, but certain abnormal 

 developments occur at times, and Gibbs himself knew of these 

 as we see on reading pages 312 and 313. Sir James Dewar 

 made many experiments on vertical films in which he observed 

 that instead of the black spreading steadily over the film, black 

 spots appeared in many places, especially at the thicker parts. 

 These spots rise to the top of the film and there coalesce to 

 produce an apparently normal black film, and the film settles 

 down thereafter to the usual course of development. This so 

 called "critical" behavior of the film seems to require some 

 definite stimulation from external sources to bring the film to 

 the state in which the "critical black fall" begins. 



