Measuring the Surface- Tension of Liquids. 411 



On several occasions water that was left in the glass dish 

 in the closed closet over night was found next morning to 

 have a surface-tension considerably lower than it had the 

 night before. If left two or three days the difference was 

 greater, even when the cover was on the dish during the 

 whole time. But after a vigorous stirring the surface-tension 

 was in each case a little greater than it had been at first. The 

 substance, whatever it was, that made the change, being 

 present in small quantity in the surface-layers of the water, 

 reduced the tension. The same amount of the substance 

 scattered throughout the whole volume of the water, and 

 hence present in very minute quantity in the surface-layers, 

 increased the surface-tension. 



Some of the purified water mentioned above was boiled 

 briskly half an hour in a covered glass beaker to expel the 

 the air. A smaller bottle was completely filled with the 

 boiled water and left to cool. Half of it was then poured into 

 the dish and its surface-tension measured without delay. 

 The remaining half was shaken up with the air in the bottle, 

 and its surface-tension measured. In the table above (2) is 

 water freed from air, (3) the portion shaken up with air. 

 The tension when air is present is greater by '06 dyne. (4) 

 and (5) are similarly related, and the difference is *07 dyne. 

 (6) was simply poured into the dish from the large bottle ; 

 and (7) was stirred vigorously in the open dish with a glass 

 rod. (8) and (9) are two specimens of clean snow melted 

 and not boiled. (10) is a portion of (9), well stirred, after 

 remaining' two weeks in a covered dish. 



The mean of these values is one dyne less than the mean of 

 Lord Rayleigh's results'* from measurements of the wave- 

 length of ripples. The difference is not accounted for by the 

 presence or absence of dissolved air, though it is probable 

 that carbon dioxide in moderate amount would make more 

 difference than the more perfect gases ; and it seems very 

 improbable that the difference is owing to constant errors in 

 either of the methods. Two possible explanations remain : 

 (1) unnoticed contaminations of the surface, and (2) real 

 differences in the surface-tension of the different specimens of 

 water. 



In order to secure a fresh clean surface I arranged a lono- 

 glass siphon to reach from a glass vessel on the shelf near the 

 balance into the glass dish in the closet below. Its lower end 

 was drawn out small and turned upward from the bottom of 

 the dish so that the incoming stream continually swept the 

 whole surface of the dish, which was tilted up at one side to 

 * Rayleigh, Phil. Mag. October 1890, p. 886. 



