CH. IV] POTOMETEE. 75 



tube. As soon as this appears the small block is replaced, 

 the tube once more dips into the water, and a bubble of air 

 included in it travels up, and serves as an index of the 

 rate of absorption. The bubble must be of uniform size in 

 successive readings, because other things being equal a long 

 and short bubble travel at different rates. To insure this, 

 mark the tube with a file at 5 or 6 mm. from its end, 

 and replace the vessel W when the air column has reached 

 the file-mark. The movement of the air-bubble is timed 

 from a mark on the tube : the upper limit of its course is 

 the upper end of the capillary tube, the moment of its 

 impinging against the column of water in the potometer 

 being easily visible. It is for this reason that the tube 

 projects above the cork, for otherwise a convenient place 

 of ending for the course of the bubble would not be 

 visible. The starting point of the course should be at 

 least 2 inches above the file-mark, so that the bubble 

 may settle down to a uniform pace before the course 

 begins; and because time is needed for the observer to 

 put down the block and the small vessel of water, and 

 take up the stop-watch. 



In this way numerous readings can be taken in a 

 short time; the air admitted collects at A, and can be 

 removed after a time ; it is obvious if the branch were 

 placed in A and the cork in G, that the admitted air might 

 diminish the surface of contact between the water and 

 the absorbing surface of the branch. 



It is well to make graphic representations of one or 

 more of the following experiments, using the reciprocals 

 of the stop-watch readings, which will be proportional to 



