242 PHYSIOLOGICAL PHYSICS. [Chap. xxn. 



atmosphere, till, in 1705, experiments made by 

 Hawksbee, at Gresham College, showed the action 

 to occur in vacuo as well as in air. The explanation 

 is that the force of attraction exerted by the walls of 

 the tube on the liquid molecules in contact with them 

 overcomes the force of cohesion of the liquid mole- 

 cules for one another, and raises the water where it is 

 in contact, causing thus the depression towards the 

 centre. As a result of this attractive force of adhesion, 

 the pressure on that part of the surface of the water 

 in contact with the tube is less than the pressure on 

 the rest of the liquid, by the amount of the force of 

 adhesion. In consequence of the diminution of pres- 

 sure the water rises in the tube, and will rise till 

 the column of water reaches a height above the rest of 

 the surface that will exert a weight equal to the force 

 of adhesion. This force of gravity, being equal and 

 opposite in direction to the force of adhesion, counter- 

 balances it, and thus the liquid comes to rest at a cer- 

 tain distance up the tube. 



It has been found that wherever the liquid wets the 

 tube the height of the capillary ascent depends on the 

 liquid, and 011 the temperature (diminishing with in- 

 creasing temperature), but not on the material of the 

 tube. With the same liquid, the extent of elevation 

 varies inversely as the diameter of the tube. That is, the 

 narrower the tube, the higher the ascent, and vice versa. 



On the other hand, if the force of cohesion is suffi- 

 cient to overcome the force of adhesion, then the liquid 

 in the tube assumes the CONVEX MENISCUS, the liquid 

 in the immediate neighbourhood of the walls of the 

 tube is depressed, and elevated towards the centre. The 

 liquid does not wet the tube. As already mentioned, 

 this is the case with mercury in a glass tube. Instead of 

 a capillary ascent of the liquid, there is a depression 

 below the level of the surface of the fluid outside of the 

 tube. This depression is explained on similar grounds to 



