240 PHYSIOLOGICAL PHYSICS. [Chap.xxn. 



mass of the mercury in its spherical shape has become 

 too great for the force of cohesion to support against 

 gravity. Consider, now, the free surface of any 

 liquid, it is easy to see that the molecules on the sur- 

 face are attracted by the molecules deeper in the fluid, 

 but have no molecular attraction beyond them. The 

 attraction is, therefore, towards the deep part of the 

 liquid. At the surface of liquids, in consequence, a 

 force is directed inwards, which is called the SURFACE 

 TENSION of the liquid. It is this phenomenon of co- 

 hesion in particular which determines the spherical 

 form of drops already noted. Some very interesting 

 experiments on surface tension may be made with 

 camphor. Drop a minute piece on the surface of 

 perfectly clean water ; it is driven about in various 

 directions, owing to the fact that the surface tension of 

 pure water is much greater than that of camphor 

 water. The solution of the camphor, therefore, 

 diminishes the surface tension in its neighbourhood, 

 and currents are produced in all directions. The 

 solution being quicker in some places than in others, 

 the strength of the currents varies, and so the frag- 

 ment is driven about in a distracted manner. If a 

 small block of charcoal be taken, and coated with 

 paraffin, and if at each end, on opposite sides, a piece 

 of the paraffin be removed, and a drop of some 

 essential oil put on the charcoal, and then the char- 

 coal be dropped on the surface of water, it will not 

 be driven to and fro, but will be turned round and 

 round, This is due to the difference of surface 

 tension between the water and essential oil at the 

 two ends on opposite sides acting as a couple. 



Adhesion of a liquid to a solid body is shown 

 when a perfectly clean piece of glass is dipped into 

 water. On removing it water is found adhering to 

 its surface. Or if a drop of water be placed on a per- 

 fectly clean glass plate, the drop of water does not 



