no TRANSPIRATION AND ASCENT OF SAP ch. 



ture rises after a time, viz., Experiments 1, 2, and 3 on 

 tube I. 



As a general rule, the temperature of rupture lowers 

 with time, suggesting that adhesion is improving. It may 

 be noted that some very high values of adhesion of water 

 to copper were accidentally obtained in tube II. , in which 

 by chance some minute shavings of copper were included. 



These experiments amply confirm Berthelot's observa- 

 tions of the tensile strength of water containing air, and they 

 raise the minor limit obtained by him of its cohesion and 

 its adhesion to glass from 50 a tin. to more than 150 atm. 

 They further show that the adhesion to the walls of the 

 conducting tracts is also over this figure. 



Tensile strength of sap. In many instances 

 sap adhering to the vegetable tissues introduced into 

 the tension tubes showed that its cohesion was no less 

 than the rest of the water, and, indeed, there was no 

 reason to suspect that it would have less tensile strength. 

 However, recently the doubt has been raised that sap, 

 especially if containing dissolved air, has not the same 

 cohesive properties as water. Consequently it seemed 

 proper to test the matter by direct experiment. 



With this end in view, the sap centrifuged from pieces of 

 branches of Fagus silvatica, cut from about 70 feet above 

 the level of the ground, was enclosed in a tension-tube. 

 This sap, after collection, was boiled on three successive days 

 for about one hour in order to secure the complete wetting 

 of dust-particles fortuitously contained in it. After its last 

 boiling it was exposed for twenty-four hours as a thin layer 

 about 4 mm. deep to the air, but shielded from dust. 

 In this way it must have become practically saturated 

 with dissolved air. The capillary tube, into which it was 

 now drawn by alternate heating and cooling, had been very 

 carefully cleaned by successive washings out with chromic 

 acid, caustic potash, and boiled water. After this cleaning 

 the tube was boiled for about an hour on three successive 



