462 Notes on the Angle of Contact. 



the difference in level between the top of the drop and the 

 mean level of the azobenzene-plate at the edges of the 

 drop. 



Particles of dust on the top of the drop were in focus over 

 a considerable area, so that the top of the drop was sensibly 

 plane. 



The necessity for tapping the plate is shown by the 

 following two readings taken without tapping : — 



Expanding drop, 0'396 cm. 

 Contracting drop, 0'270 cm. 



The angle of contact calculated from the mean value of 

 the thickness of the drop by the formulae 

 was found to be 75° 40'. 



The mean of these three independent determinations of 

 the angle of contact between water and azobenzene- is 

 approximately 77° at about 15° G. 



These experiments are being continued. 



Summary. 



1. The angle of: contact between water and solid paraffin 

 at 14° C. has been found to be 106° 2S' by direct measure- 

 ment, and 106° 57' from observing the depression of water 

 in a paraffin capillary. The discrepancy is within the limits 

 of the experimental error. 



2. A method has been devised for detecting small angles 

 of contact, and in all cases investigated a zero angle was 

 found with liquids that w T et glass. 



3. Azobenzene has been found to give a finite acute angle 

 ;ontact with water, its mean value by three methods being 



77° at about 15° C. 



We have great pleasure in thanking Professor T. P. 

 Merton, F.R.B., for much valuable advice during this in- 

 vestigation. 



Physical Chemistry Laboratory, 

 Balliol College and Trinity College", Oxford. 



