106 TRANSPIRATION AND ASCENT OF SAP ch. 



determining experimentally the pressure needed to produce 

 the same deformation, the amount of the tensile stress, 

 which was sustained by the water before rupture, was 

 determined. In an experiment, carried out in the 

 manner just described, water was subjected to a tension 

 equivalent to 7 '5 atmospheres before its cohesion was 

 overcome. 



Cohesion of a soap film. It may here be pointed 

 out that every stretched water film not only gives us a 

 demonstration of the tensile strength of water, but also 

 enables us to set to it a minor limit, which does not fall 

 far short of that determined by Berthelot. 



A film of soapy water stretched upon a rigid frame is 



stable even when it is only 12^, or 12 x 10 " 7 cm. thick. 1 



The thickness of the film is measured by the interference 



phenomena of light from the opposite surfaces of the film. 



When thus stretched, the film supports the stress of twice 



the surface tension (T) of the soap solution. This force 



has been determined as about 25 dynes per centimetre. 



Hence it follows that the film which has a cross section of 



50 x 10 

 12 x 10 " 7 sq. cm. supports a tension of 50 dynes or 



dynes per sq. cm. An atmosphere pressure is equivalent 

 to 1*0132 x 10 6 dynes. So we find, according to this method, 

 that the cohesive strength or tenacity of water must at 

 least be equal to 41 3 atmospheres. 2 



New determinations on water. Berthelot's ex- 

 periment with water enclosed in thick capillary 

 tubes, just quoted, is quite easy of repetition. As 

 some of my experiments with this method have given a 

 much higher minor limit for the tensile strength of water 

 than his, I have thought it of interest to record them here. 



The lengths of the tubes used in my experiments varied 

 between 14*5 cm. and 220 cm. 



1 In the second black film, Johonnot states the thickness may bo only (J ju/u.. 



2 I am indebted to Mr. J. lv. Cotter for this calculation. 



