on the Angle of Contact. 461 



of glass in order to ensure a plane surface. After solidifica- 

 tion, the mica was peeled off and the plates were mounted on 

 glass by placing a warm sheet of glass on the back of the 

 plate of azobenzene. 



The angle of contact was first measured by the til ting- 

 plate method. Eight readings with two plates gave a mean 

 value of 77° 20 / +40', the greatest deviation for the mean 

 being 3° 30'. As the angle is less than 90° it was necessary 

 to work with a falling meniscus in order to obtain consistent 

 results. 



As it was impossible to make a transparent tube of azo- 

 benzene, the capillary rise of water was measured, between 

 two plates of this substance separated by measured distance 

 pieces, the lower edges of the plates being immersed in water. 

 One of the plates was less than 1 mm. in thickness and 

 translucent so that the shadow of the meniscus could be seen 

 through it when illuminated with a strong light. 



The meniscus was rather irregular owing to small irregu- 

 larities in the plates, but the horizontal portions were all 

 about the same height above the surface of the water. 

 Owing to these irregularities it was impossible to observe 

 the capillary rise from the side. 



With plates O104 cm. apart, the mean capillary rise with 

 a falling meniscus was 0'31zb0'03 cm. from five determi- 

 nations. 



Assuming that T = 73 dynes/cm., (9 = 77° 30' + 1° 10', in 

 good agreement with the direct measurement. 



The angle of contact of water with azobenzene was deter- 

 mined in a third way by measuring the thickness of a large 

 drop of water standing on a horizontal plate of azobenzene. 

 The drops used had a diameter of 3 or 4 cm., and measure- 

 ments were made on both expanding and contracting drops, 

 water being either added to or removed from drops between 

 readings, and the plate tapped before taking a reading. 



The following results were obtained for the thickness of 

 drops : — 



Expanding drop 0*367 cm. 



0-321 



0-328 

 Contracting drop 0'325 



Mean 0*335 cm. 



Each value was obtained by measuring with a microscope 



