646 VICTOR ZIEGLER 



lodged and in falling will assume a more or less spherical form. 

 The smaller the drop, the more perfect its spherical shape. Here 

 we have a homely demonstration of the forces acting on the liquid. 

 The creep-up of the water on the sides of the glass is due to the 

 attraction of the glass for the water; the drop of water remaining 

 on the glass rod is held there by the same force, that is — adhesion. 

 In falling, the water from the rod does not fly off in a series 

 of small particles, but assumes a spherical shape because the 

 component particles of water, or, in other words, its molecules 

 are attracted toward each other. This is cohesion. Adhesion is the 

 attraction of unlike molecules for each other; cohesion is the 

 attraction exhibited between molecules of the same substance. 1 

 The force due to the cohesion of the molecules of different substances 

 and that due to the adhesion between the molecules of different 

 substances varies. The cohesion of water is less than its adhesion 

 for glass, hence the glass rod is enabled to tear away a certain 

 amount of water. 2 If, however, we dip a glass rod into mercury 

 and withdraw it, nothing will adhere, because, in this case, co- 

 hesion is the stronger force. 



The space through which cohesion is active is the "sphere of 

 molecular attraction. " It is a sphere about o . 00005 mm - m diame- 

 ter. 3 If we now assume that a liquid is made up of a number of 

 layers of molecules, we will see that the top layer, the free sur- 

 face, will be attracted unequally because part of its "sphere of 

 molecular attraction" lies outside the liquid. 4 



In Fig. 1 xy is the surface of the liquid. A and B represent 

 two molecules in the surface and beneath the surface respectively. 



The circles surrounding them represent 



* Q Y the "sphere of molecular attraction." 



q The molecule B is attracted equally in 



F all directions by the molecules falling 



within its sphere ; in the case of the mole- 

 cule A, however, the attraction will be downward, as the attract- 

 ing molecules only occupy that part of the sphere lying within 



1 Nichols and Franklin, Elements of Physics, 1 24. 



2 F. Pockels in Winkelman's Handbuch der Physik, I, 882. 



3 Duff, Textbook of Physics, 146. 4 Ibid., 147. 



