OBSERVATIONS ON THE DAUBREE EXPERIMENT g 



required to cause air to begin to flow through a capillary tube (or 

 pore) which originally contained water.'' 



(E) The capillary rise is affected by variation of those factors 

 which influence the angle of contact and the density and surface 

 tension of the Hquid. The changes induced in the angle of 

 contact and in density may for present purposes be neglected 

 entirely. As regards the influence of temperature on the surface 

 tension of water, all the investigations unite in showing that 

 its surface tension decreases regularly with rise of temperature, 

 becoming zero of course at the critical temperature, where there is 

 no surface of separation. The relation is practically linear when 

 the whole range is considered; it may be represented with sufficient 

 accuracy by the formula 



o-;=78 — o. 21 / or o. 21 (370— ^) 



where o-^ is the surface tension at t (temp, in Centigrade) expressed 

 in dynes per centimeter. 



The effect of pressure on surface tension is unknown, but is 

 presumably small. For the changes in the properties of water 

 induced by a pressure of, say, 1,000 atmospheres are usually similar 

 in magnitude and direction to those observed when a relatively 

 small quantity of a salt is dissolved in it; and the surface tension 

 of such dilute (0.5 N or less) solutions differs by only a few per 

 cent from that of pure water. 



Experimental. — Before proceeding to the discussion of the geo- 

 logic implications of the above principles, we shall mention the' 

 results of a few experiments on the atmometer principle, carried 

 out with cyHnders or fragments of various materials. It may be 

 mentioned that the cylinders of cement and the plaster of paris 

 were cast in glass tubes of appropriate length, which then served 



' Experiments of this kind have been made by Barus {Am. Jour. Sci., XLVIII 

 [1894], 452), by Bechhold (Zeitschr. d. physik. Chem., LXIV [1908], 328) and by 

 Bigelow and Bartell (Jour. Am. Chem. Soc, XXXI [1909], 1194). The formula 

 connecting pressure required (P, in atm.) with pore diameter {D, in millimeters) 

 is P = 0.00^04/0 (for room temperature); it is easily derived from formula 3 

 Bechhold's calculated pore diameters are tenfold too small, a fact which was noted 

 by Bigelow and Bartell. The pressure P is of course not the same as that required 

 to force water to flow through a capillary tube; for in the latter case we do not 

 necessarily have a free surface within the tube. 



