PHYSICAL PROPERTIES OF WATER 7 



The phenomenon known as Capillarity, which appears in the tendency 

 of a liquid to rise or fall from its normal level in a tube of fine bore, 

 according as the liquid does or does not wet the tube, would appear to be 

 due to cohesion and adhesion ; while it is probable that cohesion 

 between the surface molecules of a liquid gives rise to Surface Tension, 

 the property of a liquid which gives it the appearance of having an elastic 

 skin at its surface of separation from a gas, or from any other liquid. 

 Experiment shows that the tension in any such surface film is everywhere 

 the same, and that the tension across any imaginary line in the surface 

 is normal to that line. Also the tension is independent of the curvature 

 of the surface ; decreases with an increase in temperature ; is constant 

 for a given temperature, for the surface of separation of any particular 

 liquid and gas, or of any two given liquids ; and for any given liquid and 

 solid in the presence of air, the angle of inclination, a, of the surfaces is 

 constant. 



Thus, for water and glass at a temp, of 68 F. . . a = 25 32' 



for mercury and glass a = 128 52' 



while the surface tension, expressed in pounds per foot run of the line of 

 contact, and at the above temperature, has the following values (from 

 Quincke's experiments) : 



For water and air . . . T '005548 Ibs. per foot 

 For mercury and air . . . T = '03698 



No satisfactory explanation of the precise nature of surface tension has 

 been formulated. Beyond doubt, however, it is a molecular phenomenon, 

 and is inseparable from that of cohesion, the intensity of the one probably 

 determining that of the other. 



Many theories have been educed to explain the phenomena of 

 capillarity. Of these, the two which have been most generally accepted 

 are due respectively to Poisson and Young. Briefly outlined, the former 

 assumes that when a liquid wets a plate the attraction between the laver 

 of molecules in intimate contact with the plate and the plate itself is 

 greater than the intermolecular attraction of the liquid. The molecular 

 motion of the molecules forming the surface film is thus reduced, while 

 in consequence of their greater freedom, that of the molecules immediately 

 distant from this film is increased. The density of this secondary layer 

 of fluid is thus reduced below the normal, and the resultant upward 

 pressure of the surrounding fluid causes it to rise up the plate until a 

 state of statical equilibrium is attained between the cohesive forces and 

 the action of gravity on the supported fluid. 



Where a liquid does not wet the plate, the density of this secondary 



