294 PROTOPLASM 



tension of the fluid which is brought upon the solid body 

 (a ), of that of the limiting surface between this fluid and 

 the solid body {a^^, and of that of the solid body (a^), 

 which determines the spreading out of the fluid ; that is 

 to say, that the latter will continually spread out, if 

 «! — ai2>«2- Now, if we are not dealing, as was supposed 

 in the above case, with the surface of a solid body (1) 

 which is bounded by air, but with a surface covered by a 

 fluid (3), so that the fluid (2) is brought upon the limit- 

 ing surface between the solid body (1) and the fluid (3), the 

 spreading out of (2) will take place here theoretically when 

 the condition is fulfilled that ajg — a^^a,^^. The theory 

 further requires that in any drop of fluid which does not 

 spread out upon the surface of a solid object, its surface 

 should form, at each point of its line of contact with the 

 latter, a constant marginal angle with the surface of the 

 solid object — an angle which is independent of the geometrical 

 form of the surface of the drop, and is only dependent on 

 the ratio of the three surface tensions which act at every 

 point of the line of contact. 



These theoretically- obtained determinations have only 

 been partially confirmed by experience. Thus, for example, 

 the marginal angle of a drop of water ■ adhering to a glass 

 plate does not remain constant if water be gradually drawn 

 away from the drop ; the area which the drop covers 

 remains the same, while its volume diminishes, and at the 

 same time the marginal angle becomes smaller, while 

 theoretically it ought not to change, or at the most it might 

 increase, if one takes into account the decreased height of 

 the drop. Quincke was just as little successful in causing 

 an essential change in the marginal angle of an adhering 

 drop of this kind, by altering the surface tension a^^ (or a^), 

 by means of placing some oil on the drop, though he 

 succeeded perfectly with drops which were lying on the 

 surface of a fluid. In any case these results seem to 

 indicate that in adhering drops relations come into play 

 which are not yet sufficiently known to us ; for which 

 reason it seems very unsafe to make Quincke's theory the 

 basis of a hypothesis relative to the phenomena of movement 



