26 



STUDIES IN PLANT RESPIKATION AND PHOTOSYNTHESIS. 



in the mercury in the small bottle. The detail in figure 3 shows what 

 is essentially a bell-valve made of glass and sealed with mercury. 



As is shown in figure 2, the apparatus contains four of these 

 valves, each with its own electromagnet and each connecting with 

 a separate absorption tube. The time during which each valve 

 was held open represents one period of carbon-dioxid determination. 



The manner in which the electromagnets are controlled can be 

 seen by referring to figure 4. This apparatus consists essentially 



Figure 4. 

 The clock which makes electrical contact through the mercury trough and 

 thus controls the valves by means of the electromagnets. 



of a circular piece of wood 5 cm. thick and 30 cm. in diameter. 

 A circular trough has been cut in the upper face. The whole is 

 mounted on three wooden legs. In the center hole is a 24-hour 

 cylinder^clock, C. This carries a metal arm, on one end of which 

 is a mercury cup,*'and through this cup connection is made with the 

 electric current: of the^line through the wire and binding post L. 

 From the otherend of the arm is suspended a looped piece of copper 



