426 



Prof. Gr. Quincke on the Edge-angle and 



Table {continued). 



Time 



since 



plate 



was 



cleansed. 



Z. 



Time of discharge 



of the electroscope 



before ] after 



deposition of the 



drop. 



Edge- 

 angle. 



Mica (uniaxal). 





Oh. 



i'75 



06 



u 

 2 33 





Mica (biaxal). 









Oh. 

 



3 

 2 25 



175 



1 



2 15 



Saltpetre ( 1 ) 



» 1 



These experiments show that, other circumstances being 

 alike, the time of discharge of the electroscope was less as the 

 edge-angle was smaller or the surface of the solid substance 

 cleaner. 



Since the times of discharge for the same liquid were dif- 

 ferent in the different experiments, they cannot be due to the 

 deposited drops alone, but must depend upon the state of the 

 surface of the solid substance, which is modified in different 

 ways by the substances deposited upon it. 



The differences of time of discharge, however, are at once 

 explained if it be admitted that a small portion of the liquid 

 deposited spreads upon the surface of the solid substance in 

 an extremely thin film imperceptible by optical methods, and 

 that the remaining portion of the liquid rests with an edge- 

 angle > 0° upon the surface so modified or rendered impure 

 by the liquid itself. 



The edge-angle and the time of discharge were found to be 

 less, according as the thin liquid film near the flat drop of 

 liquid had a greater thickness, or as the surface of the solid 

 substance was more completely clean. 



That in the separate experiments the edge-angle and the 

 time of discharge, or the thickness of the thin film of liquid 

 near the flat drops, were found of different magnitudes cannot 

 be surprising if we contemplate the difficulties which stand in 

 the way of a complete cleansing of the plate, or if we consider 

 how the gradually augmenting impurity of the surface is the 

 result of accidentally settling particles of dust, the presence of 

 which cannot be avoided, and yet whose influence cannot be 

 allowed for. 



