800 A. W. McCOY 



Different substances have different surface tensions, which can 

 be calculated by means of formula a) with the necessary observed 

 factors. For instance, crude oil at 20 C. has an average surface 

 tension of about 25 dynes per cm.; 1 water at 18 C. about 75 dynes; 

 and mercury at 20 C. about 540 dynes. 2 



Surface tension also varies with the nature of materials in 

 surfacial contact. For instance, the surface tension of mercury 

 when in contact with water is different from when in contact with air. 

 Unfortunately, a number of such different values are not recorded, 

 so that this discussion is limited to liquids in contact with air. 



It is necessary that the adhesion of the material in the tube be 

 either greater or less than the cohesion of the liquid, otherwise 

 there would be no chance for surface tension to display itself. 

 When adhesion is less than cohesion, depression in the liquid 

 results, as in the case of mercury and glass; when adhesion is 

 greater than cohesion, there is a rise in the capillary tube. If 

 adhesion greatly overbalances surface tension, the liquid surface 

 may break and the liquid mount up the sides of the vessel, as in 

 the case of some light oils in a low porcelain cup. Consequently, 

 before one liquid will replace another in capillary openings the 

 replacing liquid must not only have a greater surface tension but 

 also a greater adhesive power for the material of which the tube is 

 composed. 



Capillary force according to equation a) is a function of surface 

 tension, contact angle, diameter of pore space, density of liquid and 

 acceleration of gravity. In the case of water-air surface the con- 

 tact angle is o, therefore (cos a) equals 1 ; the density of water is 

 1; so the equation resolves itself into: 



h = kT/r, 



where k equals 0.00204. 



Starting with a temperature of 15 C, at a depth of 100 m., the 

 capillary pressures shown on p. 801 are computed from the above 

 formula. Pressures are recorded in kilograms per square centimeter. 



The following calculations show, first, that capillary pressures 

 decrease with depth on account of the increase in temperature; 



1 Washburn, A.I.M.E., L, 831. a Tait, Properties of Matter, p. 264. 



