Optical Test for Angles of Contact. 28i) 



As the microscope was fitted with verniers for both hori- 

 zontal and vertical movements, the readings were easily 

 taken. The method is, of course, not capable of great 

 accuracy, owing to the changes taking place in the drop due 

 to evaporation. In the case of a liquid which evaporates 

 quickly (e. g. turpentine) the readings must be taken very 

 rapidly. 



The following are some measurements taken in this way, 

 which appear to be sufficiently close to show that the argument 

 is correct : — 





Water. 





« = -644 



b = -oU h=-392 



T = 75-6 



•764 



•673 -350 



78-2 



'932 



•66 -294 

 Turpentine. 



73-5 



a =-674 



b=. -331 A = \L51 



T=26-9 



•576 



•396 -15 



28-2 



•661 



•354 -148 



27-4 



Glycerine. 



a =-443 b= -287 h = -370 T = 74-9 



•435 -344 -337 75*2 



•358 -400 -344 75-5 



•367 -338 -392 74-4 



•308 -369 -382 74-5 



a, b, h are in cm., T in dynes per cm. It will be noticed 

 that though the method gives values for water and turpentine 

 in the neighbourhood of those generally accepted, the values 

 for glycerine, though consistent with each other, are too high. 

 This may be due to the fact that the glycerine drop is much 

 thicker than either of the others, and the formula for thin 

 lenses may not apply; or it may be due to the rapid absorption 

 of aqueous vapour from the air. 



University College, Galway. 



Phil. Mag. S. 6. Vol. 31. No. IU. April 1916. X 



