GROWTH 221 



that will be absorbed by the liquid in the basin of the 

 apparatus B. To sum up : between the plant and the 

 apparatus B there will be a similar circulation of 

 matter as between a plant and an animal. The plant 

 under the bell will be provided with a periodical 

 automatic source of carbonic acid. The plant will thus 

 be supplied with carbonic acid until the little marble 

 ball is entirely dissolved, a matter of days and weeks. 

 Without coming into contact with the marble, the 

 plant is thus able to nibble it all up, and use its carbonic 

 acid for food.^ But in order to know whether every- 

 thing is right under the bell, i.e. that carbonic acid is 

 being given off and decomposed by the plant, the 

 apparatus B is supplied with the following adjustment. 

 Into the twice bent tube C (shown in greater detail in 

 fig. 64, C) a drop of mercury is introduced. Evidently 

 whenever, owing to the expansion of the air b (fitg. B), 

 the level of the acid in the funnel rises from m to n, the 

 level of mercury in the tube C will likewise rise from 

 m' to n' (fig. C). Two insulated conductors joined to a 

 common electric bell are introduced into the open end 

 of this tube. One of the conductors is always immersed 

 in the mercury, the other stops a little higher up, 

 and comes into contact with the mercury only when 

 the latter rises to n'. The moment this happens the 

 electric circuit is closed, and the bell rings. It is pretty 

 easy to regulate the apparatus in such a way that the 

 giving off of carbonic acid will happen only when there 

 is very little of it left under the bell, and that the bell 

 will always ring whenever carbonic acid is given off. 

 Just as the giving off of carbonic acid ceases when the 

 acid returns to its former level m, so the bell ceases to 

 ring when the drop of mercury returns to its former 

 position and opens the circuit. But if, owing to some 



"■ By weighing this little ball from time to time we can determine 

 approximately the amount of carbonic acid absorbed by the plant in so 

 many days or weeks. 



