CHAPTER V 



TENSILE STRENGTH OF THE SAP OF TREES 



Even in text-books of Physics the cohesion of liquids 

 is seldom discussed, and the conditions necessary to pro- 

 duce a state in which liquids may transmit a tensile stress 

 are not adequately treated. 



Cohesion of liquids. Donny in 1846 showed that 

 it was possible for a column of sulphuric acid 

 1255 m. high to hang in a vertical tube closed at 

 its upper end, when atmospheric pressure was not 

 allowed to press the liquid upwards from below. He 

 compares the phenomenon to the well-known experience 

 that the mercury of a barometer may be retained above the 

 actual barometric height, if the tube, filled by inclining it, 

 is raised gradually to a vertical position. He further states 

 that this phenomenon has been explained by Laplace as 

 being due to the cohesion of the mercury and to its adhesion 

 to the glass. Donny also looked for the cohesion or tensile 

 strength of water. He appears, however, to have failed to 

 demonstrate it in the same way in which he had successfully 

 showed it in the case of sulphuric acid. He observed, how- 

 ever, the tensile strength of water in the following less direct 

 manner : If a vertical glass tube one metre long, partially 

 filled with water and sealed at both ends, is struck vigor- 

 ously on the lower end with the palm of the hand, bubbles 

 open in the liquid and instantly close again with a metallic 



