86 TRANSPIRATION AND ASCENT OF SAP ch. 



If now, by gradually inclining the tube, the long limb 

 is completely filled with water (Fig. 16 B) and all the bubbles 

 are chased out of that limb by holding the bent end upper- 

 most, so that no breaks, even the most minute, remain, 

 we shall find, on inverting the tube, that the water 

 remains in the long limb and does not, under the force 

 of gravity, take up the lowest possible level in both limbs 

 (Fig. 16 C). From the level in the two limbs it is evident 

 that the hydrostatic pressure of the shorter column cannot 

 possibly balance the pressure of the column in the longer 

 limb ; the one is about 85 cm. higher than the other. The 

 water in this case hangs in the tube. The liquid in the 

 long limb is in contact with the glass all over, and, since 

 it wets it perfectly, it adheres to it. To the film of water 

 adhering to the glass the rest of the water coheres, and this 

 cohesion is well able to sustain the weight of the column 

 of water which is counterbalanced by no other upholding 

 force. In this way the lower part of the water in the 

 longer limb of the tube transmits a stress through the 

 upper part to the glass equivalent to its gravitational 

 pull. 



The reality of this pull becomes all the more evident 

 when, by destroying the cohesion at one spot, a rupture is 

 started. This rupture, which may at first be invisibly 

 small, rapidly spreads across the whole column. The 

 rupture may usually be started by a sharp knock admini- 

 stered to the side of the longer limb ; but, when the 

 cohesion is very perfect, to produce a rupture may require 

 a shock so violent as to be liable to shatter the tube. 

 When the rupture is started, the lower part tears suddenly 

 away from the upper part of the column and falls into the 

 bend of the tube. The upper part follows it more slowly, 

 trickling down the inside of the tube, and all the water 

 comes to occupy a position in the lower part of the tube 

 (Fig. 16 A). 



It is instructive to note how the cohesion of the water 



