Dixon — Note on the Tensile Strength of Water. 61 



hand. At tlie moment of the shock, bubbles open in tlie water and close 

 immediately with a metallic click. When a tube, exactly similar to the last, 

 but exhausted of air, is treated in the same way, no bubbles are developed, 

 nor is the click lieard. Donny explains that, in the first case, tlie blow causes 

 minute bubbles in the liquid to be opened against their surface-tension forces 

 and the air-pressure in the tube, and that therefore, in the second case, where 

 the bubbles are not formed, the cohesion must be greater than these two 

 forces taken together. Donny believed that even a very little air dissolved 

 in the water suffices to reduce the tenacity of water to a vanishingly small 

 figure. 



He also points out that the boiling of liquids is retai'ded when air is 

 removed from them, owing to their increased cohesion ; and it is the cohesion 

 being suddenly overcome which causes explosive boiling. 



Berthelot,^ a few years afterwards, succeeded in showing very simply that 

 water has a very considerable cohesion, and, under proper conditions, can 

 sustain a very great tensile stress. His experiment has been so often 

 misquoted that it may be well to quote his own description of his method : — 

 " Si Ton remplit d'eau a la temperature de 28 ou 30 degres un tube eapillaire 

 un pen fort, ferme par un bout et termine de I'autre par une pointe effilee ; si 

 Ton refroidit ce tube jusqu'a 18 degres, de facon a y faire rentrer une certaine 

 qtiantite d'air par la pointe ouverte ; si alors on le ferme et qu'on chauffe de 

 nouveau jusqu'a 28 degres, et graduellement au-dessus, au bout de certain 

 temps I'air se dissout completement. Si Ton refroidit a 18 degres temperature 

 initiale a laquelle le tube renfermait a la fois du gaz et du liquide, on 

 remarque que I'eau continue a occuper la totalite de la capaeite interieure, et 

 conserve ainsi une densite invariable de 28 a 18 degres .... 



" La variation de densite ainsi produite est enorme : pour I'eau elle est 

 egale a -^^-Q de son volume a 18 degres . . . Un semblable eifet, pour se produire 

 eu sens eontraire, exigerait une pression d'environ 50 atmospheres pour I'eau." 



Dr. Joly and the author- also carried out some experiments on the 

 behaviour of stressed water in presence of air. These experiments were 

 designed to find out if water adhered to the conducting tubes of plants suffi- 

 ciently vigorously to transmit the stress needed to raise the _transpiratiou 

 stream. Also we were misled by erroneous quotations from Berthelot's paper 

 into believing that this investigator had only experimented with air-free water. 

 As was anticipated, we found that water containing large quantities of air in 



' M. Berthelot, Sur quelques phenomenes de dilatation foreee des liquides. Ann. de Chimie 

 et de Physique, xxx., 1850, pp. 232 et seq. 



2 Dixon and Joly, On the Ascent of Sap. Phil. Trans., Eoy. Soc, vol. 186 (1895), B. 



