Contact-Angle of Liquids and Solids. 181 



Angle zero. 



Angle finite. 



Value of o>. 



Ethyl-alcohol. 



Water (?) . . 



. Small. 



Methyl-alcohol. 



Acetic acid . 



. 20°. 



Chloroform. 



Turpentine . . . 



17°. 



Formic acid. 



Petroleum . 



26°. 



Benzine. 



Ether . . . . 



16°. 



Discussion of 'the Results. — The close agreement of the " q" 

 and u k" results in those cases in which no finite contact- 

 angle is inferred, by showing the general validity of the 

 method of experimentation, is such as to force me to rely 

 with some confidence on the evidence of the existence of a 

 finite contact-angle in the other cases. It is more difficult to 

 decide if this contact-angle is due to the relations among the 

 molecular forces in equilibrium at the line of contact, as was 

 assumed, in the statement of the problem, would be the case. 

 Another possible explanation may be given, in favour of 

 which the experiments here presented offer some evidence. 

 In experiments conducted on the plan here described, the 

 temperature of the liquid is almost necessarily a little lower 

 than that of the air above the lens. Presumably the lens, 

 the upper surface of which is not covered by the liquid, is 

 also at a slightly higher temperature than the liquid. The 

 evaporation from its under surface will therefore be more 

 rapid than from the cooler lower surface of the bubble. After 

 the interior of the bubble has become saturated with the 

 vapour of the liquid, evaporation will go on from the lens and 

 precipitation will occur upon the surface of the bubble. This 

 evaporation will cut away the thin liquid surface at its line of 

 contact with the glass; and unless the facility with which the 

 liquid travels over glass is such that new particles move up 

 to completely supply the deficiency, there will not be perfect 

 replacement of the surface, and it will meet the glass with a 

 finite contact-angle. The marked difference between the 

 " q" and " h" results of March 7 for the values of a 2 for 

 water, indicating a large contact-angle, taken in connexion 

 with the large difference between the temperature of the 

 water and that of the air, points towards the explanation just 

 given as a possible one. The liquids, too, for which no finite 

 contact-angle was indicated are either not very volatile or are 

 extremely mobile, and bubbles formed in them move freely 

 under the lens. Babbles in those liquids for which a finite 

 contact-angle appears are, in every case except that of ether, 

 much less free in their movements under the lens. The 

 extreme volatility of the ether may explain the presence of 



