v TENSILE STRENGTH OF SAP OF TREES 103 



surface-tension forces capable of supporting the hydro- 

 static head of the liquid below, will not destroy the tensile 

 state. 



Berthelot's estimate. 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 procedure was as follows : He rilled 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. 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 solution 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. 

 From this he argued that the water preserved the same 

 density from 28 to 18. The dilatation needed to effect 

 this is very large, viz., l-420th of its volume at 18. 

 To produce a similar effect in the opposite sense would 

 require a pressure of about 50 atm. ; and it was concluded 

 that the experiment showed that neither the adhesion to 

 the glass nor the cohesion of the water is less than 50 atm. 



Berthelot's experiment has been variously misquoted, 

 (1) with regard to the dilatation observed, and (2) as to 

 the effect of dissolved air on the tensile strength of water. 

 The dilatation has been quoted as amounting to 1-1 20th 

 of the volume instead of 1 -420th. This, of course, gives 

 a much too high result for the tension obtained in the 

 experiment. Hence Ewart's quotation of Berthelot in 

 support of the statement that air-free water can sustain 

 a tension of 200 atm. was illegitimate. The minor limit 

 obtained in Berthelot's experiment was 50 and not 200 atm. 



Again, it is quite usual, when treating of the cohesion 

 of liquids, to state that Berthelot's experiments were 



