Transpiration and the Ascent of Sap. 35 



tube and preserved in this way the same density from 28 '^ — 18". 

 The dilatation needed to effect this is very large, viz., for water 

 '/42„ of its volume at 18" C. To produce a similar effect in the 

 opposite sense would require a pressure of about 50 atm. 



The experiment shows that neither the adhesion to the glass nor 

 the cohesion of the water is less than 50 atm. 



Berth elot's experiment has been variously misquoted (1) with 

 regard to the dilatation observed and (2) as to the affect of dissolved 

 air on the tensile strength of water. The dilatation ^) has been quoted 

 as amounting to 7i2o of the volume instead of ^420- This of course 

 gives a much too high result for the tension obtained in the experi- 

 ment. Hence E wart'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 Berth elot'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 carried out with air- 

 free liquids. As a matter of fact his method of experiment shows that the 

 water contained air, and he expressly states that the water was, in his 

 first experiments, supersaturated, and that it was only at Re gn an It's 

 suggestion that he carried out experiments on air-free water. His 

 final conclusion is "Le phénomène (dilatation of water under tension) 

 se produit donc dans le vide aussi bien que dans l'air, et est in- 

 dépendant de la sursaturation". 



Misled by the misquotations just alluded to, although à priori 

 there seemed no reason to suspect that the presence of dissolved air 

 would weaken the tensile strength of water, Dr. J 1 y and the 

 author^) considered it necessary to investigate the point specially. 



We used a cylindrical glass vessel with rounded ends and pro- 

 vided at one end with a narrow tubulure. This vessel was very 

 carefully cleansed by washing it internally successively with caustic 

 potash solution, dilute acid and distilled water. Half filled with water 

 it was boiled for some time to make sure that the walls were thoroughly 

 wetted; then it was almost completely filled with water which had 

 been previously boiled to get rid of undissolved air and to thoroughly 

 wet all dust particles which might have been contained in the liquid. By 



^) Poynting and J. J. Thomson, Text Book of Physics: Properties of 

 Matter. 2nd Ed., p. 123. 



-) A. J. Ewart, The Eesistance to Flow in Wood Vessels. Ann. of Bot., 

 1908, p. 444. 



') A. M. Worthington, On the Mechanical Stretching of Liquids. Phil. 

 Trans. Roy. Soc. Lond., Vol. 183, (1892), A, p. 355. P y n t i n g and J. J. T h m p s n , 

 loc. cit. 



*) H. H. Dixon and J. Joly, On the Ascent of Sap. Phil. Trans. Roy. Soc. 

 Lond., Vol. 186, (1895), B, pp. 568 et seq. 



3* 



