144 A TREATISE ON METAMORPHISM. 



in general 0.00005 mm." Plateau made the distance through which mole- 

 cular attraction acts 17 q 00 mm., 6 which amount is slightly greater than 

 Quincke's determination. Since each wall holds a film of water, sheet pas- 

 sages below 0.0001 mm. in diameter are subcapillary. The maximum size 

 for the subcapillary circular openings is twice as great, or 0.0002 mm. in 

 diameter. 



The laws of flowage of water through tubes of such small size have 

 not been investigated, so far as I am aware. However, upon theoretical 

 grounds one would expect that the flow would be exceedingly, indeed 

 indefinitely, slow even as compared with flow in capillary tubes. This 

 anticipation is fully justified by the observed facts of geology. It is well 

 known that natural oil and gas may be held in anticlinal arches and domes 

 for long periods of time, even when under great pressure. It is certain in 

 these cases that the escape of oil, or even gas, through the subcapillary 

 openings of the shales is slower than the manufacture of these products in 

 nature's laboratory. The facts as to the retention of oil and gas under 

 shale roofs render it highly probable that flow in subcapillary openings 

 is so slow as to be inappreciable during the time through which an experi- 

 ment is ordinarily continued; but the flow in subcapillaiy openings during 

 geological periods is probably of great consequence. (See pp. 892-904.) 



It may be anticipated that the slow movement of water in subcapillary 

 openings is greatly influenced by change of temperature. At high temper- 

 atures the viscosity of water is an important element in flow, and this 

 rapidly decreases with increasing temperature. That water gas does not 

 obey the law of flow of liquids in subcapillary tubes is shown by the 

 experiment of Daubree," in which the vapor of water at a temperature of 

 160° C, and consequently at a pressure of 6 atmospheres, passed through 

 a layer of apparently solid rock 2 cm. in thickness, and gave a pressure on 

 the other side of 1.9 atmospheres. This experiment shows beyond all 

 question that water gas under high pressure and temperature does not 

 adhere to the Avails strongly, and has such a small viscosity that it slowly 

 but surely passes through subcapillary openings. However, ground water 

 at all temperatures below the critical temperature under ordinary conditions 



"Quincke, M., Ueber die Entfernung in welcher die Moleeularkrafte der Capillaritat noch wirk- 

 sam sind: Poggendorff, Annalen, vol. 138, p. 402. 



b Plateau, J., Statique des liquides, vol. 1, 1873, p. 210. 



•=Daubree, A., Geologie experimental, Paris, 1879, vol. 1, pp. 236-238. 



