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PLANT PHYSIOLOGY 



Second are manometer tubes, containing mercury in a U-shaped bend which 

 is pushed down one arm and up another by the pressure. These are of 

 two types, open and closed as to the distal end. The former are the more 

 striking and easy of manipulation, but they require a larger 

 quantity of water than is always availatle, and moreover if 

 the pressures are considerable, must be of a very inconvenient 

 and break -tempting length. Hence the latter or closed type 

 are far superior for such purposes as the present. The best 

 form I have been able to develop, one which gives correct 

 readings with very small quantities of liquid, is among my 

 normal apparatus (page 46) and is shown by the accompanying 

 figure (Fig. 42). It is made of small-bore (.5 mm. or less) 

 barometer tubing, of the form, relative length of arms, and 

 position of reservoir bulb shown in the figure, these details 

 being of consequence to its ready filling. The long arm is 

 graduated from above downwards to admit of ready computa- 

 tion of the air column, and the short arm is provided with three 

 glass sleeves, which serve, when cemented over one another and 

 over the gauge with sealing-wax or shellac, to make the gauge 

 large enough for ready attachment to larger stems. The gauge, 

 after being cleaned and dried by aid of alcohol, is filled thus: 

 the short arm is placed in boiled (air-free) water, and clean dry 

 mercury is admitted through the longer arm until the gauge is 

 filled from end to end, when air is admitted above and the 

 mercury allowed to find its natural 

 level. Then a piece of closed 

 rubber tubing (that through which 

 mercury is admitted, or a pipette 

 bulb) is placed on the upper end, 

 and, by a quick compression, the 

 air column is forced down around 

 the bend to the bulb and allowed 

 to spring as quickly back, when 

 a part of the mercury will be 

 forced out and replaced by water, 

 which should fill somewhat less 

 than half of the bulb. Next the 

 air in the long arm is to be dried 

 (for presence of vapor-tension 

 causes an error difficult to estimate) ; 

 a closed rubber tube or pipette bulb FlG - 4 2 -— High-pkesstoe manometer; Xi- 

 is filled with water and placed over the lower end, where it is gently pressed 

 so as to force the air out through a rubber tube into a chamber containing 

 calcium chloride and phosphorus pentoxide, into and out of which it is moved 

 a few times when it becomes thoroughly dry. The connection is then sev- 

 ered, the exact height of the vertical mercury column, the room temperature, 

 and (for exact work) the barometric pressure are carefully noted, and the 



