TRANSPIRATION AND ASCENT OF SAP DIXON. 415 



began, the water and the glass commenced to contract. The coeffi- 

 cient of expansion for heat of water being greater than that of glass, 

 the water tended to contract more. This contraction, however, was 

 resisted by its adhesion to the glass and its own cohesion, and conse- 

 quently a stress or tension, which kept it sufficiently dilated to fill 

 the glass, was set up. As cooling proceeded the tension grew greater 

 and greater, till at last either the adhesion or cohesion was overcome 

 and a break appeared either between the water and the glass or in 

 the substance of the water itself. The appearance of this rupture 

 was signalized by a sharp click, and a bubble sprang into existence 

 in the water. The bubble thus produced rapidly augmented in size 

 as the water, now relieved from the stretching forces, assumed a 

 volume corresponding to its temperature at the moment. Bubbles 

 appear around the original bubble and pass into it. 



By estimating the amount of deformation x of the glass envelope 

 when strained by the contracting water, and by determining experi- 

 mentally 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 man- 

 ner just described, water was subjected to a tension or pull equivalent 

 to 7.5 atmospheres before its cohesion was overcome. 



As was noticed this method of showing the cohesive property of 

 water is precarious — the slighest 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 

 fulfills these conditions. 2 



The vessel in which the liquid is to be inclosed is a J-shaped glass 

 tube about 1 centimeter in diameter (see fig. 2). The long limb of 

 the J is about 90 centimeters while the shorter one is about 20 centi- 

 meters long. On the shorter limb there is a bulb with a capacity of 

 about 60 cubic centimeters. 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 described in the preceding experi- 

 ment and about 100 cubic centimeters of repeatedly boiled water is 

 introduced into it. In order to be certain that the glass is thor- 

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

 contact with any dust particles contained 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 



1 H. H. Dixon and J. Joly, On the Ascent of Sap. Phil. Trans. Roy. Soc London vol 

 186 (1895), B, p. 569. 



2 H. H. Dixon, Physics of the Transpiration Current. Notes from the Botanical School, 

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



