32 PLANT RESPIRATION 



manner one can easily work, by continual interchange of 

 tubes, as long as one pleases without interruption. Fig. 5 

 represents a complete Pettenkoffer apparatus in action. The 

 Pettenkoffer tubes which have been cut out are emptied by 

 shaking into dry 150 cc. glass bottles with ground and greased 

 glass stoppers. The bottles are allowed to stand quietly until 

 the next day in order that the precipitate may settle completely. 

 Then 25 cc. of clear baryta solution are measured out with a 

 pipette from each bottle, for titration. 



Ordinary aspirators are used for the production of the air 

 current. It is very practical not to join the aspirator directly 

 to the water supply but to a vessel of large capacity placed as 

 high as possible, filled from the tap and fitted with a siphon. 

 The latter arrangement prevents an overflow of the vessel. In 

 this way there is always a weak but constant water pressure. 

 A considerable pressure is unnecessary to draw the air through 

 the absorption vessels. The possible uncertainty of the water 

 pressure in the water supply could also be entirely avoided by 

 using Palladin's mercury regulator. This simple device should 

 be used for all work in which a continual current of gas is desired 

 and a constant supervision is not always possible. The regu- 

 lator is set up in the following manner. A layer of mercury 

 about 2 cm. deep is poured into a triple-necked bottle and on 

 it poured a layer of water of the same depth. The entry tube 

 is fitted into one opening and the open end immersed i cm. in 

 the water. The exit tube passes out through the other opening 

 but in the middle one is an upright glass tube whose lower 

 end dips in the mercury by i cm. (Fig. 5). The regulator 

 is inserted in the air current between the absorption vessels and 

 the air pump. By the use of this regulator the rarefaction of 

 the air in the apparatus can reach at the most i cm. of mercury 

 pressure, since the air breaks in to the pump through the 

 middle tube of the regulator in case the passage of gas is 

 obstructed or entirely blocked. In this way the interchange of 

 absorption apparatus may be accomplished without stopping 

 the action of the air pump. 



The series of various vessels for the passage of air is then as 

 follows: I. Vessel with soda lime to absorb the atmospheric 



