INTRODUCTION . 13 



full to the brim, i.e. until the water-level is as high as 

 the edge of the tumbler. Then we say that the 

 tumbler is full of water. But it will still hold a good 

 deal more without spilling. Add water by teaspoon- 

 fuls, or drop coins in, one by one. The level rises 

 well above the edge, and still if due care is taken, no 

 water is spilt. The tumbler can be made over-full 

 of water. It is plain that something restrains the 

 natural tendency of water to seek the lowest level, 

 and to escape by flowing over the edge. 



Can it be that the water is sensibly viscous, like 

 glycerine or thick treacle .-* This can hardly be the 

 explanation, for the excess of water does not level 

 down gradually, but takes up a position of rest in 

 less time than it costs us to focus the eye upon it. 

 It is not the want of fluidity in the water which 

 prevents it from flowing over the edge. There is a 

 film overspreading the surface, even of the purest 

 water, which prevents it from escaping until the 

 pressure becomes considerable. 



Dip your finger into water and lift it out again. 

 The water does not all run off, but flows down to the 

 tip of the finger, and there forms a big drop. What 

 holds up the drop .-* It is the surface-film, which does 

 not differ in its composition from the water within, 

 but is for the moment in a peculiar physical state. 

 The film is incredibly thin. It can be renewed in an 

 instant, fresh water-particles taking its place, while 

 the old film loses all its peculiar features, and mingles 

 with the water beneath the surface. 



The surface-film can be made evident in another 

 way. Take a clean needle, poise it horizontally on 



