Djxon — On the Tensile Strength of Sap. 231 



with dissolved air. The capillary tube, into which it was now drawn by 

 alternate heating and cooling, had been very carefully cleaned with a 

 succession of chromic acid, caustic potash, and boiled water. After this 

 cleaning, the tube was boiled in water for about an hour on three successive 

 days, heating and cooling being effected in the same water. The tube was 

 emptied before eacli boiling, and allowed to fill with the freshly boiled water. 

 The object of this was to thoroughly wet the tube, and any dust-particles it 

 contained, by bringing all undissolved air on their surfaces into solution. 

 The tube after filling with the sap to within a few millimetres of its end was 

 sealed off. Tlie heating of the tube was effected, as in my previous work, 

 in a large volume of water, and was very slow. 



In the first tube submitted to experiment the air-bubble disappeared at 

 63'50° C, which may be described as tlie " closing " temperature, and 

 reappeared with the characteristic click at 59-10°. Three other observations 

 were made with this tube. All four agree in indicating that the sap with- 

 stood a tension of over 45 atmospheres before rupture (cf. Expts. 1, 2, 3, and 

 4 in the Table). 



A second tube was charged with some of the same sample of sap ; it was 



found to become completely filled at 66-25° C, and ruptured at 59-50° 0. 



Calculating the tension developed in this case, the result is over 70 atmospheres 



■ (see Experiment 5 in the Table). In another experiment with this tube a 



tension of about 50 atmospheres was produced (see No. 6). 



It was thought that possibly, by keeping one of these tubes after closing 

 at a temperature close to that at which the bubble disappeared, greater 

 tensions might be attained. This surmise was not realized. The tube 

 used in the first experiments described above was kept for two days at a 

 temperature of about 61° C. However, when ultimately allowed to cool 

 slowly, the rupture occurred at 59-20°, a temperature not quite so low as had 

 sometimes before been successfully passed. This experiment is recorded as 

 No. 3 in the Table. 



It may be noted that there is no reason to believe that the tensions 

 produced in these experiments are indications of the maximum cohesion of 

 boiled sap. The results quoted happen to be the first obtained. Other 

 experiments were not made, as these are sufficient to demonstrate that the 

 boiled sap possesses cohesive properties of the same order as those of water. 



Having found that sap, free from unwetted nuclei, but saturated with 

 air, is able to sustain considerable tensions, it seemed worth while trying if 

 unboiled sap could be put into the tensile condition. The consideration that 

 heating the enclosed sap in the glass envelope until the last visible bubble 

 disappeared would probably completely remove all invisible bubbles 



