CAPILLARY ACTION. 



567 



If the tension of the surface between the solid and one of the fluids ex- 

 ceeds the sum of the other two tensions, the point of contact will not be in 

 equilibrium, but will be dragged towards the side on which the tension is 

 greatest. If the quantity of the first fluid is small it will stand in a drop 

 on the surface of the solid without wetting it. If the quantity of the second 

 fluid is small it will spread itself over the surface and wet the solid. The 

 angle of contact of the first fluid is 180 and that of the second is zero. 



If a drop of alcohol be made to touch one side of a drop of oil on a 

 glass plate, the alcohol will appear to chase the oil over the plate, and if a 

 drop of water and a drop of bisulphide of carbon be placed in contact in a 

 horizontal capillary tube, the bisulphide of carbon will chase the water along 

 the tube. In both cases the liquids move in the direction in which the surface- 

 pressure at the solid is least. 



ON THE RISE OF A LIQUID IN A TUBE. 



Let a tube (fig. 5) whose internal radius is r, made of a solid substance c, 

 be dipped into a liquid a. Let us suppose that the angle of contact for this 

 liquid with the solid c is an acute angle. This 

 implies that the tension of the free surface of 

 the solid c is greater than that of the surface 

 of contact of the solid with the liquid a. Now 

 consider the tension of the free surface of the 

 liquid a. All round its edge there is a tension 

 T acting at an angle a with the vertical. The 

 circumference of the edge is 2nr, so that the 

 resultant of this tension is a force ZirrTco&a 

 acting vertically upwards on the liquid. Hence 

 the liquid will rise in the tube till the weight of the vertical column between 

 the free surface and the level of the liquid in the vessel balances the resultant 

 of the surface-tension. The upper surface of this column is not level, so that 

 the height of the column cannot be directly measured, but let us assume that 

 h is the mean height of the column, that is to say, the height of a column 

 of equal weight, but with a flat top. Then if r is the radius of the tube at 

 the top of the column, the volume of the suspended column is irr'h, and its 



Fig. 5. 



