SIZE OF OPENINGS IN ROCKS. 135 



openings the ordinary laws of hydrostatics apply. Capillary openings for 

 water solutions include those which, if circular tubes, are smaller than 

 0.508 mm. in diameter, and those which, if sheet spaces, are narrower than 

 0.254 mm., and which in either case are larger than the openings in which 

 the molecular attractions of the solid material extend across the space. 

 Such openings in the case of circular tubes are those smaller than 0.0002 mm. 

 in diameter, or, if sheet passages, are below 0.0001 mm. in width. Capil- 

 lary openings, therefore, include circular tubes from 0.508 to 0.0002 mm. 

 in diameter, and sheet passages from 0.254 to 0.0001 mm. in width. Capil- 

 lary openings of other forms have a range limited between 0.508 and 

 0.0001 mm., but no one form has so wide a range as this. To movement 

 of water in openings such as these the laws of capillary flow apply. By 

 subcapillary openings are meant those in which the attraction of the solid 

 molecules extends from wall to wall. These include all tubes smaller than 

 0.0002 mm. in diameter, and sheet, openings smaller than 0.0001 mm. in 

 width. For intermediate forms the subcapillary openings have as their 

 maximum limit a range from 0.0002 to 0.0001 mm. 



It is not supposed that supercapillary openings, capillary openings, 

 and subcapillary openings are sharply separated from one another. They 

 grade into one another, and the laws below given which control the flowage 

 in one class of openings are gradually modified until they pass into the 

 laws which control the flowage in another class of openings. For instance, 

 water in circular tubes slightly larger than 0.508 mm. in diameter would 

 to some extent obey the laws of flowage of capillary openings, and water 

 in tubes slightly less than 0.508 mm. in diameter would to some extent 

 obey the laws of supercapillary flow. In short, flowage in openings near 

 the dividing line between two classes obeys laws intermediate between 

 those controlling flowage in the typical cases of each class. 



The areas of openings of variable size and similar form vary as the 

 squares of their respective diameters. The circumferences of openings of 

 variable size and similar form vary as their respective diameters. It follows, 

 for a given volume of water, that the larger the openings in which it is 

 contained the less is the surface of contact. For instance, if for an opening 

 of any form, of given diameter, the surface of contact for 1 cm. of length 

 be 1 sq. cm., if the cross diameter be doubled, the length remaining the 

 same, the volume of the water is four times as great, but the surface of 



