136 A TREATISE ON METAMORPHISM. 



contact is only twice as great. If the diameters be decreased to one-third, 

 the volume of the water is decreased to one-ninth, but the surface of contact 

 to one-third only. 



As a consequence of the relation between size of opening's and area of 

 contact, it follows that in small openings a given volume of water is capable 

 of performing much more work upon the rocks than in openings of larger 

 size, for the surfaces of contact are the places where chemical interaction 

 between the water and rock takes place. How important is the factor of 

 small size in the amount of work which may be accomplished by ground 

 water can be adequately comprehended only when the surface of action for 

 a given volume of water for small openings is calculated. To illustrate, if 

 the openings are circular tubes of a size at the border line between those of 

 supercapillary and capillary size — that is, tubes 0.508 mm. in diameter — 1 

 cu. cm. of water would have a surface contact with the rocks of about 78.74 

 sq cm. If the openings be sheet openings at the boundary between super- 

 capillary and capillar)- — that is, 0.254 mm. in width — 1 cu. cm. of water 

 would haA'e a surface contact of about 78.74 sq. cm. If the openings be 

 circular tubes at the border line between those of capillary and subcapillary 

 openings — that is, 0.0002 mm. in diameter — 1 cu. cm. of water would have 

 a surface contact of about 200,000 sq. cm. If the openings be sheet open- 

 ings at the border line between those of capillary and subcapillary size — 

 that is, have a width of 0.0001 mm. — 1 cu. cm. of water would have a 

 surface contact of 200,000 sq. cm. Therefore 1 cu. cm., or 1 gram of water, 

 has a surface contact varying from 0.007874 to 20 square meters in circular 

 capillary tubes; and in sheet passages has a surface contact varying from 

 0.0U7874 to 20 square meters. It has been calculated by Whitney that 

 "the grains in a cubic foot of soil have, on the average, no less than 50,000 

 square feet of surface area,"" The magnitude of these numbers shows how 

 important a factor in the work of a given volume of ground water is the 

 size of the openings in which the water is contained. 



It follows from the above relations that the area of contact, and 

 therefore the friction between moving water and the fixed film of water 

 adherent to the walls, is inversely as the size of the openings. As will be 



« Whitney, Milton, The physical principles of soils in their relations to moisture and crop distri- 

 bution: Bull. Weather Bureau No. 4, IT. S. Dept. of Agric, 1892, p. 14. 



