IV 



COHESION THEORY OF ASCENT OF SAP 85 



necessary to bring this property of liquids into play is 

 by performing the following experiment. 



The vessel in which the liquid is to be enclosed is a J- 

 shaped glass tube about 1 cm. in diameter (see Fig. 16). 

 The long limb of the J is about 90 cm., while the shorter 

 one is about 20 cm. long. On the shorter limb there is 

 a bulb with a capacity of about 60 c.c. The shorter limb 

 is continued beyond the bulb as a narrow tube drawn out 

 to a point. The whole tube is carefully washed out in 

 the manner to be described in the following chapter, and 

 about 100 c.c. of repeatedly boiled water is introduced 

 into it. In order to be certain that the glass is thoroughly 

 wetted, and also to make sure that the water is in perfect 

 contact with any dust particles con- 

 tained in it, the liquid is again 

 repeatedly boiled after introduction 

 into the tube. Before sealing off the 

 fine tube the whole of the space 

 unoccupied by the liquid is filled 

 with steam by bringing the water to 

 ebullition, and, when the steam has 

 expelled the air, and is issuing 

 through the narrow tube the latter 

 is sealed off. When the whole has 

 cooled, it will be found that the 



J -tube acts as a water-hammer, i.e., if, by inclining the 

 tube the water is made to travel from end to end, its 

 concussion makes a metallic ring. This is owing to the 

 fact that very little air has been included when the tube 

 was sealed, and water-vapour at normal temperatures is 

 unable to act as an elastic pad in the same way as air at 

 normal atmospheric pressure would. The clicking metallic 

 ring, then, may be taken as an indication that the gas- 

 pressure within the tube is very slight. Care, indeed, 

 must be taken not to let the concussion become too violent, 

 as in that way the tube may be easily shattered. 



