on the Angle of Contact. 459 



nearest the observer within about 0*5 mm. of the side of the 

 cell, so that the surface of the water at the edge of the cell 

 was above the general level and did not impede observation. 



In order to facilitate the detection of small curvatures in 

 the surface, a strip of white paper with a horizontal straight 

 edge was pasted along the back of the cell, with its upper 

 edge about 1 mm. below the water surface. The strip was 

 illuminated from the side, and a black background was used. 

 By observing the upper edge of the paper in juxtaposition 

 to its reflexion in the surface, quite small curvatures of the 

 surface could be detected. 



Five slides were used, and eight readings were taken with 

 each, four on each side. The average deviation from the 

 mean was 31/, the greatest deviation being 77'. 



The means of the readings with the five slides were : — 



106° 33' 

 106° 41' 

 106° 18' 

 106° 21' 

 106° 25' 



Mean 106° 28' 



This result shows satisfactory agreement with the indirect 

 determination (106° 57') considering the experimental error 

 in the latter case. The value found by the direct method is 

 of course the more reliable of the two. 



The value of the surface tension of water at 14° C. found 

 by combining the value of 9 found directly with the mean 

 value of T cos found in the preceding experiment, is 

 75*5 dynes/cm. 



Determination of the Angle of Contact between Glass and 

 certain Liquids. 



An attempt was made to measure the angle of contact 

 between glass and various liquids by means of a glass plate 

 which could be tilted by the apparatus described above. 

 The liquids selected were water, acetic acid, turpentine, and 

 ether, which were thought by Magie to give finite angles of 

 contact. 



It was soon found that the tilting-plate method was 

 unsuitable for the measurement of small angles of contact, 

 and it was abandoned in favour of an apparatus designed to 

 detect very small angles of contact. This consisted of a 



