20 SCIENCE PROGRESS. 



and no air beyond the current under investigation is ever 

 admitted to the chamber, except such as has been carefully 

 freed from CO,. 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, 

 gave 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 GO, when working on re- 

 spiration, or when working on assimilation through one for 

 adding C0 2 . 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 CO, 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 constant rate. 

 Thus a constant amount of CO, is being continually generated 

 and is carried off by the air current. The amountof 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 satisfactorily. 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 re- 

 ceivers are of various forms, according to the material ex- 

 perimented on, but are all constructed on the cardinal 

 principle of making them as small as possible consistent 

 with the well-being of the part, in order that changes in the 

 composition of the gas shall, as soon as possible, be felt by 

 the current which passes thence through narrow tubes to 



