28 PLANT RESPIRATION 



sterilized, nutrient solution plus 12% gelatine. After the 

 substrate had set, a pure culture was poured in and by rotation 

 [of the cylinder] cUstributed evenly over the walls of the vessel 

 (Fig. 4)- 



The above examples suffice to show how varied the containers 

 can be. In all types the inner openings of the entry and the 

 exit tubes are as far removed from one another as possible. 

 ' „ This is always a necessary precaution to prevent 



lUL an accumulation of CO2 in the container. 



As for the absorption apparatus, it can likewise 

 be varied. If a more common type of absorption 

 vessel is not required, but the amount of COo 

 developed considerable, the Geissler potash tubes 

 or soda lime tubes may be used to advantage. 

 The bulbs of the potash tubes are about two- 

 tliirds filled with 40-45% caustic potash. The 

 Fig. 4.— Roll ^se of concentrated potash solution is not to be 



culture. 



recommended, for a stoppage of the tubes often 

 results from the precipitation of crystalline potassium hydrox- 

 ide.' The calcium chloride tube of the Geissler apparatus is 

 filled with solid calcium hydrate, or still better, with asbestos 

 soaked in concentrated sulphuric acid, to prevent a loss of 

 water. Both ends of the tube are plugged with glass wool. 



The potash apparatus thus prepared provides for a complete 

 absorption of CO2 even if a rapid stream of pure carbon dioxide 

 is passed through it. In this case one can easily make sure that 

 the stream of gas passes through the second bulb only in single 

 small bubbles but that it does not pass the third bulb. The gas 

 which passes the potash apparatus must be entirely dry how- 

 ever. To provide for this a wash bottle filled mth concentrated 

 sulphuric acid is interposed before the apparatus. This pre- 

 caution is entirely sufficient while the use of calcium chloride 

 introduces experimental errors, since potassium hydrate absorbs 

 water vapor as does calcium chloride. The difference is really 

 unimportant with short exposures but can be quite marked in 

 experiments of an hour's duration. 



' There is nothing to prevent the use of sodium hj-dro.\ide in place of the 

 potash solution. — Ed. 



