TRANSPIRATION AND ASCENT OF SAP DIXON. 413 



creased proportionately to the increased tension. But it is open to 

 many of the objections which overthrew the imbibition hypothesis, 

 viz, the lumina are known to transmit the major part of the current, 

 and it seems improbable, even where we can invoke such great forces 

 as the tensile strength of water, that they could suffice to drag an 

 adequate water supply through the fine-grained cell walls. Yet we 

 were able to show by experiment that even when the lumina are 

 rendered impassable for water, some small amount of water is trans- 

 mitted in the walls by this process. 



When w^e found ourselves compelled to give up these hypotheses, 

 the one, as assuming conditions inimical to the transmission of ten- 

 sion in the water, and the other, because it did not agree with the 

 ascertained fact that the water moved in the lumina, it was an easy 

 transition to arrive at the conclusion that the water passed up in the 

 lumina in a state of tension. How, in the lumina of the conducting 

 wood, the necessary conditions for the production of tension are ful- 

 filled, we shall now proceed to enquire. 



Even in textbooks of physics the cohesion of liquids is seldom 

 alluded to, and the conditions necessary to produce a state in which 

 liquids may transmit a tensile stress are not adequately treated. 



Donny in 1846 showed that it was possible for a column of sul- 

 phuric acid 1.255 millimeters high to hang in a vertical tube closed 

 at its upper end, when atmospheric pressure was not allowed to press 

 the liquid upward from below. He compares the phenomenon to the 

 well-known experience that the mercury of a barometer may be re- 

 tained above the actual barometric height if the tube, filled by in- 

 clining it, is raised gradually to a vertical position. He further states 

 that this phenomenon has been explained by Laplace as being due to 

 the cohesion of the mercury and to its adhesion to the glass. * * * 



Berthelot^ a few years afterwards succeeded in showing directly 

 that water has a very considerable cohesive strength and, under 

 proper conditions, can sustain a very great tensile stress. His pro- 

 cedure was as follows: He filled a strong capillary tube, which was 

 sealed at one end and drawn to a fine point at the other, with water 

 at a temperature of 28° or 30° C. He allowed it to cool to 18°, and, 

 as it cooled, to draw in air. Then the fine-drawn end was sealed. 

 The tube was now^ heated to 28° or over, and the air forced into solu- 

 tion in the water, which now occupied the whole of the internal space 

 of the tube. On cooling to 18° or lower it was found that the liquid 

 continued to occupy the entire space enclosed by the tube and pre- 

 served in this way the same density from 28° to 18°. The dilatation 

 needed to effect this is very large, viz, for water one four-hun- 

 dred-and-twentieth of its volume at 18° C. To produce a similar 



iM. Berthelot, Sur quelques phenom&nes de dilatation forcee des liquides. Ann. 

 Chim. et de Phys., vol. 30, 1850, pp. 232 et seq. 



