62 



EXPERIMENTS WITH PLANTS 



an inch and a half of the top with fresh syrup. Fit 

 a small rubber cork to the tube and push it down 

 about half an inch into the tube; insert a 

 needle at one side of the cork, so as to allow 

 the compressed air to escape and restore the 

 normal pressure. Withdraw the needle and 

 carefully dry out the inside of the tube 

 above the cork. Melt some sealing-wax in 

 a spoon, and pour it slowly into the tube 

 until it runs over. With a hot knife smear 

 it over the outside as far down as the cork, 

 so as to close the tube air-tight (see Fig. 52) . 

 Obtain or prepare a strip of paper ruled in 

 fine divisions (millimeters or twenty -fourths 

 of an inch, if possible), and gum it to the 

 upper part of the tube for the purpose of 

 measuring the length of the air -column in 

 the tube. Place the tube in water, note the 

 length of the air- column, and observe it fre- 

 quently during the experiment. The amount 

 of pressure produced in the tube can be 

 easily calculated from the amount of com- 

 pression which the air undergoes. The for- 

 mula for the calculation is : 



52. Apparatus 

 for measur- 

 ing the pres- 

 sure due to 

 osmosis. 



Length of air-column at start = Pressure at finish. 

 Length of air-column at finish Pressure at start. 



Thus, if the column measure one inch at 



