Measuring the Surface-Tension of Liquids. 409 



existence of a finite contact-angle. The maximum -weight 

 method is free from both these objections, and can be used 

 for almost any liquid. If the specific gravity of the liquid is 

 not accurately known, a thin frame should be used, so that 

 an error in the specific gravity will have small influence on 

 the final result. For accurate results great care must be 

 taken to prevent any oscillation of the balance as the weight 

 approaches the maximum. Zero weighings are also essential. 



III. Surface-Tension of Water. 



In spite of the labours of a large number of investigators, 

 the snrface-tension of water is still more or less uncertain. 

 Apart from experimental errors, which are seldom less than 

 one half of one per cent., there is very great difficulty in 

 retaining a pure water surface throughout the measurement. 

 Using the methods of measurement described in Part II. of 

 this paper, it became evident at once that the problem of find- 

 ing the surface-tension of water accurately, to one tenth of 

 one per cent, or less, was practically reduced to that of securing 

 a clean surface and a constant temperature for pure water. 



The weights used were found consistent with each other, 

 and therefore presumably correct, to one tenth of a milligram, 

 with two or three exceptions which were taken note of. The 

 arms of the balance were sensibly equal, and the riders 

 accurate. One of the mercury thermometers was graduated 

 in half degrees, Centigrade, from —10° to +100° ; the other 

 in fifths of a degree, from —4° to +30°. The latter could be 

 easily read to fiftieths. The zeros of both were correct. 



Some water that had been distilled repeatedly by Dr. Loeb 

 in such a manner as to remove carbon dioxide and organic 

 matter and ammonia, was kindly furnished by him for 

 measurement of the surface-tension. I preserved it in a large 

 well-washed glass bottle whose stopper and neck were covered 

 by an inverted beaker to keep off dust. 



The deep evaporating-dish already mentioned, its cover 

 the thermometer, and the glass frames (fig. 1) were well washed 

 in a dilute solution of caustic soda and thoroughly rinsed 

 first in ordinary distilled water and secondly in some of the 

 water that was to be examined. The ground-glass cover 

 remained on the dish except while frames were being put in 

 or taken out ; and before each measurement the temperature 

 of the whole was kept nearly constant for half an hour or 

 more. 



The observations extended over two days, partly because of 

 the difficulty there was in getting a film to last long enough 

 to be weighed. On the third day the surface-tension was *so 



