RESPIRATION AND FERMENTATION 



I2 5 



tube, the gas of the jar can be driven through the tube, now undamped 

 (and preferably having a capillary point ensuring small bubbles and better 

 absorption), into the bottom of a tall vessel of clear lime-water, an arrange- 

 ment more effective if tried beside a control lacking the seeds. Another 

 very effective lecture demonstration is afforded by use of the normal respi- 

 rometer later described (page 129); after two or three days' growth of the 

 seedlings therein, the rubber tube, first clamped near the measuring-tube 

 (to keep clean the mercury in the reservoir tube), is cut (by scissors) under 



A B 



FIG. 31. DEMONSTRATION EESPIROSCOPES; Xt- 

 Explanation in text. 



the surface of a solution of strong caustic potash, when the mercury will 

 run out and be replaced by the potash, which will rise in a striking manner 

 as it absorbs the carbon dioxide. 



ERRONEOUS EXPERIMENTS. Several errors or fallacies are current in 

 connection with experimentation upon this subject. One of these has already 

 been noted, viz., that the rise of a carbon dioxide absorbing solution in a 

 respiroscope tube shows that carbon dioxide is released by the seeds. Another 

 of a different sort is the assumption thai mercury rising in a tube as carbon 

 dioxide is absorbed by caustic potash in the chamber, marks the extent of 

 the absorption. In fact the mercury is so heavy that the air in the vessel 

 is rarified, the more so the higher it rises. Hence the absorption is much 

 greater than the mercury indicates, though by an amount easily calculated 

 (see correction table in Part III). The same objection applies of course 

 to caustic potash in the open tube, but to a degree so much less as to be negligi- 



