CH. Il] GAS ANALYSIS. 43 



column is at a sufficient height, the tube is temporarily 

 closed with a clip and afterwards more securely by a bit 

 of glass rod R, whose lower surface is ground flat and 

 greased, so that when pushed home it fits close against 

 the ground surface of B. 



The height of the column of mercury is now read 

 off on the calibrated tube, and at the same time the 

 height of the little column of water is recorded. Readings 

 of the barometer and thermometer are also taken. From 

 8 to 10 c.c. of COo which has been washed in NaHCOg 

 to free it from HCl is now passed into the tube, and 

 the readings are again taken. Before introducing the 

 CO2 its purity should be tested by ascertaining that 

 it is entirely absorbed over KHO. The apparatus is now 

 exposed to bright diffused light for 5 or 6 hours, or it 

 may be exposed to sunlight. When the exposure to light 

 is complete the leaf must be pulled out by the wire, and 

 when the apparatus has cooled, readings are again taken. 



In order to estimate the quantity of CO2 which has 

 been decomposed, about 0*2 or 0'3 c.c. of concentrated 

 KHO is injected into the gas-tube ; this Pfeffer recom- 

 mends to be done by the heat of the hand acting on a 

 closed pipette. After 2 hours the COo may be assumed 

 to be all absorbed, when readings are again to be taken. 

 The volume of the leaf is also to be ascertained by 

 sinking it in a narrow measuring glass and reading off 

 the altered position of the level ; the fluid in the glass 

 may be a mixture of alcohol and water which prevents 

 adhesion of bubbles to the leaf. The volume of the leaf 

 being known it must be applied as a correction to the 



