CH. Il] PHOSPHORUS METHOD. 41 



(on which as Engehiiann states he founded his method) the 

 blood must be considerably diluted, and again exhausted. 

 A sprig of Elodea is placed in a corked test-tube (see note 

 3, p. 51) and exposed to bright light with a control test- 

 tube. The blood which contains the plant shows an arterial 

 tint while the control blood remains venous. 



(49) BoussingauWs phosphorus method^. 



Fill a bell-jar over water with hydrogen and add a 

 small proportion of COo, i.e. not more than 8 per cent, of 

 the volume. Introduce a stick of phosphorus and a 

 leafy branch. The oxygen in the intercellular spaces of 

 the plant will attack the phosphorus, and the bell-jar will 

 be filled with white fumes. The bell-jar must therefore 

 be placed in the dark for two or three hours, or until the 

 white fumes are dissolved in the water, and the contents 

 of the jar are clear and transparent. The bell-jar is now 

 exposed to the sun, when in a few minutes it becomes 

 clouded with white fumes. We find that, when replaced 

 in the dark, a quarter of an hour is sufficient for the 

 absorption of the fumes. 



(50) Pfeffers method-, 



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

 taining a known volume of COo : after a certain number 

 of hours the amount of COo 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 



1 See Deherain, Chimie Agricole, p. 82. 



- Sachs' Arbeiten, i. p. 15. See also Pfeffer's Physiolorjie, i. p. 188. 



