v TENSILE STRENGTH OF SAP OF TREES 109 



In Table 10 are detailed the experiments on eight 

 different tubes, and in it are recorded the radii R 

 and r, the observed temperature when the tubes were 

 full L, and the temperature at which the rupture took 

 place t 1} together with a, the coefficient of expansion 

 for water over the range (t^ t^, and finally the tension 

 calculated according to the above formula. 



In making the observations on the temperatures, the 

 tubes were set in a large beaker of water. The tempera- 

 ture at which the tube became filled was then roughly 

 determined by warming the beaker up very slowly till the 

 bubble in the tube disappeared. The tube was then allowed 

 to cool, and the bubble to reappear. The beaker was again 

 raised to a temperature one degree below that at which 

 it was expected the bubble would disappear. The beaker 

 was kept at this temperature for five or ten minutes, and 

 the water within it was kept in motion to secure a fairly 

 uniform temperature. Fifteen minutes were occupied in 

 raising the water of the beaker through the next degree, 

 so that the water in the capillary tube must have very 

 closely approximated to the temperature indicated by the 

 thermometer in the beaker. By proceeding in this way 

 every effort was made to avoid exaggerating the tempera- 

 ture at which the tube filled. The large amount of 

 water in the beaker secured that the cooling should be 

 extremely slow before the reappearance of the bubble, 

 so that it is improbable that the thermometer gave 

 readings sensibly different from the temperature of the 

 tube. 



In spite of these precautions, differences are observed 

 in the successive experiments with the same tube. These 

 can scarcely be due to errors of observation. It generally 

 happened that when a tube was heated on successive days, 

 lower readings were obtained for the " full " temperatures 

 in the later observations. See Experiments 4, 5, and 6 

 on tube II. Occasionally, however, the " full " tempera- 



