v TENSILE STRENGTH OF SAP OF TREES 107 



In each case the tubes were first cleaned with a solution 

 of caustic potash, which was afterwards removed by 

 repeated rinsing with boiled, distilled water. A piece 

 of the wood of the yew (Taxus baccata) was then 

 introduced, and, after being filled with boiling water, 

 the tube was kept submerged in boiling water for 

 an hour or so. Before sealing the drawn-out end, 

 the water was allowed to cool ; and a millimetre or 

 more of the bore was cleared of water by warming the 

 point in a flame. When all was cool, the fine end was 

 sealed. 



In the table given on p. 108 is a record of my experiments ; 

 for each experiment the temperature t 2 at which the tube 

 is completely filled, and the temperature t v at which 

 rupture took place, are given in the fifth and sixth columns 

 respectively. 



From these observations the tension may be roughly 

 determined according to the formula 



a = JL Zizli 



where /3 = the coefficient of compressibility of water, V^ = 

 volume under the lower pressure P l5 V, = volume under the 

 increased pressure P 2 . A tension = P, will bring about the 

 same change of volume in the opposite sense. Evidently the 

 apparent change of volume of the water due to this tension 

 must be corrected for the contraction of the glass cooling 

 through the range of the experiment. The correction is 

 allowed for by the formula 



t- ( a -f/)(^'-i) 



~/3[l-a(* 2 -*!>]' 

 where 



T = tension ; 



a = coefficient of expansion of water over the range ; 



;/ = coefficient of cubic expansion of glass = 2 "4 x 10 



t l = temperature of rupture ; 



t 2 temperature when tube is full. 



