1894.] on Vegetable Assimilation and Respiration. 163 



while, if desired, estimations of the C0 2 evolved in respiration by 

 some other part can be carried on simultaneously in a separate cur- 

 rent of air freed from C0 2 . This is made possible by the apparatus 

 being practically in duplicate ; strictly comparative experiments can 

 thus be carried out. 



The actual estimation of the C0 2 is accomplished by the well- 

 known method of absorption by baryta solution and titration with 

 hydrochloric acid. The novelty consists in this, that only a very 

 small quantity of baryta solution (under 15 c.c.) is employed in each 

 experiment, and that after the absorption the whole of this is titrated 

 with acid in the tube in which the absorption has taken place. 

 Further, the burettes containing the standard solutions are alwavs in 

 air-tight connection with this absorption chamber, and no air beyond 

 the current under investigation is ever admitted to the chamber, 

 except such as has been carefully freed from C0 2 . The special 

 arrangements for stirring and emptying, by means of this air, freed 

 from C0 2 , and stored under pressure, cannot be entered into here. 



The two currents of air passing continuously through the apparatus 

 are generated by two aspirators of a special type, which, worked on 

 the principle of Mariotte's bottle, give a practically constant outflow 

 in drops, whatever the level of the water within them, and are 

 adapted to work steadily with small rates of flow (50 to 100 c.c. 

 per hour). These currents enter the apparatus, either through an 

 arrangement for removing the C0 2 when working on respiration, 

 or when working on assimilation through one for adding CO;. 

 Both these are so constructed that the current never has to bubble 

 through a layer of liquid, and so is supplied to the plant at strictly 

 atmospheric pressure, thus avoiding any risk of drawing gases 

 mechanically out of the part under investigation. The remover of 

 C0 2 is a " tower " full of beads, over which a stream of strong potash 

 flows continuously, and through which the air is drawn. The C0 2 

 generator is constructed on a new principle, and consists of a tall 

 tube containing fragments of marble, through which the air current 

 passes at a constant rate, while very dilute HC1 trickles down it at an 

 extremely slow rate, which is made constant and independent of 

 external variations of temperature by special arrangements. Thus a 

 constant amount of C0 2 is being continually generated, and is carried 

 off by the air current. The amount of C0 2 formed can be controlled 

 by the strength of the acid employed. When generating amounts 

 below 2 per cent, of the air current, this arrangement works very 

 constantly. From the C0 2 generator or remover, as the case may be, 

 the current of air passes to the receivers, in which the parts of the 

 plant under investigation are situated. These receivers are of various 

 forms, according to the material experimented on, but are all con- 

 structed on the cardinal principle of making them as small as 



w 2 



