Surfaces, Clean and Contaminated. 



397 



indicated. This is the view adopted by Quincke, who in an im- 

 portant series of observations* has shown that the edge angle 

 between water and glass has frequently a considerable value de- 

 pendent upon theimpurity of glass surfaces, even when carefully 

 cleaned by ordinary methods. But I confess that the argument 

 does not appear to me conclusive. The angles recorded are maxi- 

 mum angles. If after a drop has been deposited some of the 

 liquid is drawn off, the angle may be diminished almost to 

 zero. Observations upon capillary heights correspond surely 

 to the latter, condition of things, for no experimenter measures 

 the gradual rise of liquid in a dry tube. I am disposed to think 

 that the assumption = is legitimate, and thus that the lower 

 value of T is really supported by experiments of this class. 



Leaving now the results for pure surfaces, let us pass on to 

 those found for water contaminated with grease up to the point 

 where the camphor scrapings were judged to be " very nearly 

 dead." It must be remembered that the additions of oil were 

 discontinuous, and that the point could not always be hit with 

 precision. On any one day it is possible to set up a fairly 

 precise standard of what one means by " very nearly dead ;" 

 but the standard is liable to vary in one's own mind, and is of 

 course impossible to communicate to another. Too much 

 importance therefore must not be ascribed to exact agreement 

 or the failure of it. On one day experiments were made by 

 varying the areas enclosed within the hoop. Thus, if the 

 motions were a little too lively, they could be deadened to the 

 required point by contraction of the area and consequent 

 concentration of grease. This procedure was not so con- 

 venient as had been hoped, in consequence of the mechanical 

 disturbance attending a motion of the hoop. In all cases an 

 observation, for the most part recorded in the previous table, 

 was made first upon a clean surface, so as to ensure that the 

 contamination was all of the kind intended. The results arc 

 collected in the annexed table : — 



Date. 



Water. 



Oil. 



Tension. 



Kemarks. 



June 30 



Tap 



Olive-oil 



537 





July 1 



j» 



>> 



511 





2 



„ 





521 





4 



Distilled 





53-0 





7 



5> 



,, 



53-0 



Not quite independent. 



11 



,, 



, 



53-0 





29 



Tap 



Oleic Acid 



53-6 





Aug. 2 



„ 



,, 



53-6 





2 



„ 



Olive-oil 



524 





2 



" 



" 



52-4 



Another sample. 



* Wied. Ann. 1877, vol. ii. p. 145. 

 Phil. Mag. S. 5. Vol. 30. No. 186. Nov. 1890. 



2E 



