CHAP. VI.] THE WATER FILM loi 



out the wax will drain away for a short time before 

 setting to a solid state. Oil or wax is only used because 

 it exaggerates and renders more perceptible the results 

 which would be equally given by water ; the appearances 

 presented are shown in the photograph. It will be 

 found that all the beads are covered with a skin of oil 

 or wax, just as they would have been " wetted " had 

 they been dipped in water, but this skin is thinnest on 

 the upper row of beads, and gets perceptibly thicker 

 on each lower row. Moreover, wherever two beads are 

 in contact there is a much thicker layer of liquid in the 

 angle joining the two surfaces, though again the 

 accumulations are greater the lower the row; finally, 

 the lowest row of beads carry beneath them large drops 

 which are just not big enough to fall off. The whole 

 set of beads must be considered as enclosed within a film 

 of oil, which we may regard not so much as sticking to 

 the material of the beads but as drawn over them, 

 because the outer surface of the film acts like an 

 elastic skin — so that we might compare the arrange- 

 ment to a set of balls tied up in a stretched sheet of 

 thin indiarubber. This stretched skin on the surface 

 of a liquid is always pulling inward and trying to shrink 

 into the smallest possible area ; for this reason drops of 

 water assume a round shape whenever they reach such 

 a small size that their weight becomes less effective than 

 the elastic pull of the film, or the surface tension as we 

 shall call it. To take another illustration : the hairs of a 

 dry paint-brush stand out to form a round end ; wet 

 them, and they are dragged together to a point by the 

 surface tension of the film of water trying to reduce 

 itself to the smallest possible area. It is not merely the 

 contact with the water which causes the pointed end, 

 because when the brush is dipped below the surface it 

 retains the rounded shape it possesses when dry, the 



