CH. l] SUCCULENTS. 13 



has been proved to be steadily above that of the surround- 

 ing air, the atmosphere in the balloon is replaced by 

 hydrogen, for which purpose the two lateral apertures 

 will serve. The readings of the two thermometers should 

 now become practically equal, and according to Pfeffer it 

 is possible to re-establish the difference by readmitting 

 air. The experiment is a difficult one and should only be 

 attempted by a student of some experience. 



(11) Succulents 1 . 



In certain succulents an increase of the acidity of the 

 cell sap is accompanied by a fixation of oxygen. 



A leaf of Rocltea falcata is taken from the plant at the 

 close of a hot summer-day, cut into pieces and introduced 

 into a graduated gas-tube of simple test-tube form : it 

 may be kept in place by a plug of glass-wool. It is not 

 necessary to stand the tube in mercury, water will serve 

 quite well. We have also employed an arrangement like 

 that given in fig. 1, the KHO being omitted and water 

 replacing the mercury in the beaker Hg. The apparatus 

 is kept in the dark until the following morning, when a 

 considerable rise in the water column is visible. As a 

 control a similar graduated tube is fitted up with non- 

 succulents, such as pieces of young sunflower 2 . 



1 See De Saussnre, Eecherclies Chimiques, (An. xii = 1804), p. 65; 

 also Detmer, Praktikum, p. 224, whose arrangement of the experiment 

 we have adopted. 



2 To complete the experiment, the relative acidity of the Rochea in 

 the evening and next morning should be compared. See Part n. 



