CH. ll] GAS ANALYSIS. 35 



(50) A demonstration method recommended by 

 Deherain^ is the following. A current of air is drawn by 

 means of an aspirator through a glass tube, which is 

 carefully lined (paved as it were) with leaves, and exposed 

 to bright light. The air, after slowly traversing the tube, 

 is made to bubble through baryta water. If the current 

 of air is kept slow the baryta water is said to remain 

 clear while a current of the same rapidity which has 

 passed through an empty tube clouds the solution. 



(51) Pfeffers methods 



A leaf is exposed to light in a calibrated tube con- 

 taining a known volume of CO2 : after a certain number 

 of hours the amount of CO2 decomposed is estimated by 

 absorbing what remains with KHO. The tube is almost 

 •36 cm. in length, of which 26 cm. is a calibrated tube of 

 14 — 15 mm. in diameter, T (fig. 6) ; above this part the 

 tube is blown into a balloon and ends above in a narrow 

 tube B with flat ground edges. The whole tube contains 

 about 120 c.c. The leaf to be experimented on is rolled 

 into a cylinder and gently pushed up the tube with a 

 wooden rod until it reaches the wide part of the tube 

 where it unfolds of itself. After the experiment is over, 

 the leaf is to be removed by means of a piece of thin 

 iron wire, W, attached to the stalk before the leaf was 

 inserted. The wire should be attached outside the tube 

 by an elastic band E. The tube is fixed vertically in a 

 glass beaker, H, having upright sides and containing 



'■ Chimie Agricole, p. 75. 



^ Sachs' Arbeiten, i. p. 15. See also Pfeffer's Physiologie, i. p. 188. 



3—2 



