34 



this water we place a little alcohol. Immecliately the Avater 

 rushes away and stands round in a circular ridge like a hollow 

 crater ! It leaves the middle almost, if not entirely, bare of 

 liquid, save that some of the alcohol at least will not take part in 

 the flight. The tension of the pure alcohol is 25, while that of 

 pure water is 81. Hence the surface film is set in motion from 

 the alcohol towards the water all round, and motion ensues of so 

 energetic a character as to leave a bare place where the drop of 

 alcohol was put. A dimple may be formed on the surface of 

 water by holding a brush or small sponge dipped in ether or 

 alcohol just above the water surface. This retreat of water 

 from the presence of alcohol, which I am treating as a physical 

 fact with no moral significance, may be observed in an ordinary 

 tumbler. If in the middle of the surface of a glass of water 

 a small quantity of alcohol be gently introduced a rapid 

 rush of the surface occurs outwards from the place where 

 the spirit is introduced. If the sides of the glass be wet 

 with water above the level of the surface, and the spirit be 

 introduced in sufficient quantity, the fluid may be .seen to 

 -ascend the sides of the glass, accumulate in certain places, 

 and fall down again into the mass of the liquid. Wine contains 

 alcohol and water, and when it is exposed to air the alcohol 

 evaporates faster than the water. In a deep vessel like a wine 

 glass, this produces little eftect at first in the mass of the wine, 

 but a good deal in the thin layer of wine which, if the glass be 

 disturbed, exists on the sides of the glass above the liquid 

 surface ; tension being greater in the most watery parts, it pulls 

 itself literally together away from the alcoholic parts where the 

 tension is less. These ridges flow down the sides by their own 

 weight. As they slide down they may sometimes be seen to stop 

 and retreat when they come into contact with the alcohol. So 

 the process goes on until in time there is very little alcohol left 

 in the wine glass." 



The surface tension of liquids is loAvered as their tempera- 

 ture rises, until, at their boiling point, it is reduced to nil, and 

 they are reduced to vapour. If 100 drops of water be collected 

 from the hot-water tap, and 100 drops from the cold, it will be 

 found that the amount of water obtained from the cold tap 

 exceeds the amount from the hot tap, the reason being that the 

 hot films break more quickly, not being able to support such 

 large drops. 



A little practical application of th^ laws of surface tension 

 is useful in getting rid of drops of oil from clothing. If a ring 

 pf turpentine, ether, or benzol, whose surface tension is very low, 

 be made round the grease spot, the grease immediately retreats 

 on to its centre, where it may be absorbed by blotting paper. Or 

 the grease spot may be " chased about" by threatening it with a 



