102 TRANSPIRATION AND ASCENT OF SAP ch. 



click. A blow on a tube, which has been similarly set up, 

 but from which the air has been removed by careful ex- 

 haustion, produces no bubbles ; nor is a click heard. 

 Donny explained that in the first case the blow causes 

 minute bubbles to be opened against the forces of surface 

 tension and of atmospheric pressure, therefore, in the 

 second case where bubbles are not formed the cohesion 

 must be greater than these two forces together. Donny 

 believed that even a little air in solution suffices to reduce 

 the cohesion of a liquid to an insensible figure. This error, 

 soon to be corrected, has been frequently copied by writers 

 on this subject. Donny also pointed out that the boiling 

 of liquids is retarded when air is removed, owing to their 

 increased cohesion, and it is the sudden rupture of the 

 liquid which causes explosive boiling. 



As will presently be made clear, the absence of dissolved 

 air from the water is a condition by no means necessary 

 for its cohesion, and in these experiments it appeared neces- 

 sary only because by the removal of the dissolved air perfect 

 contact with the glass and complete wetting of the dust 

 particles suspended in the liquid was secured. 



In his memoir Donny l points out that when one with- 

 draws a plane disc from contact with a surface of water 

 the tensile strength of the latter does not come into play. 

 As the disc is raised, water adheres to its lower surface, 

 but the column of water connecting the disc with the 

 liquid below grows gradually thinner, until at a moment 

 when the disc is removed a certain distance above the 

 general level of the lower liquid, the column spontaneously 

 draws in from the edges, and, when its diameter becomes 

 extremely small, breaks in two. He shows also that, in 

 a tensile liquid column, a bubble, sufficiently small to have 



1 The presence of undissolved air, or unwetted surfaces, or both, probably 

 prevented Janse from obtaining considerable tensions in the experiment 

 quoted by him : " Der attfsteigende Strom in der Pflanze," Jahrb. f. M iss. 

 Bat., 1908, 45, 3, p. 314. 



