Transpiration and the Ascent of Sap. 



37 



mentalh' 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 or pull 

 equivalent to lb atmospheres before its cohesion was overcome. 



As was noticed this method of showing the cohesive property of 

 water is precarious — the slightest overheating- is liable to burst the 

 glass vessel containing the water. It is convenient therefore to have 

 a more simple method of demonstrating this . 

 property, which may be repeated as often 

 as is desired without risk. The following 

 method fulfils these conditions ^) : 



The vessel in which the liquid is to 

 be enclosed is a J-shaped glass tube about 

 1 cm in diameter (see Fig. 3). The long 

 limb of the J is about 90 cms while the 

 shorter one is about 20 cms long. On the 

 shorter limb there is a bulb with a capacity 

 of about 60 CCS. The shorter limb is con- 

 tinued beyond the bulb as a narrow tube 

 drawn out to a point. The whole tube 

 is carefully washed out in the manner 

 described in the preceding experiment and 

 about 100 CCS 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 

 contained in it, the liquid is again re~ 

 peatedly 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 



Fig. 3. Tube for showing the 

 cohesion of water. A posi- 

 tion assumed by the contained 

 water when the tube is sup- 

 ported vertically and when 

 cohesion is not acting. B in- 

 clined position for tilling the 

 long arm of the tube. C posi- 

 tion in which the water is 

 held, when its cohesion pre- 

 vents its falling into the bend 

 of the tube. 



') H. H. Dixon, Physics of the Transpiration Current. Notes from the Botanical 

 School, Trinity College Dublin, No. 2, 1897, p. 5. 



