106 



Messrs. F. L. Usher and J. H. Priestley. [Apr. 13, 



small but significant amount of oxygen can still be evolved from a plant 

 which has been chloroformed and subsequently exposed to light in presence 

 of carbon dioxide, the experiment has been repeated in a different form and 

 under more rigorous conditions. As it was essential that no trace of air 

 should remain in the experimental vessel, the latter was exhausted very 

 thoroughly with a small Topler pump. Fig. 1 shows an arrangement which 

 was found convenient in all such experiments. 



The plant was contained in a wide glass tube A, which was then drawn 

 out at the open end and sealed to a piece of narrow tubing thickened to a 

 capillary, at a. This tube was sealed to a T-piece leading to the pump, and 

 carrying a tube B containing precipitated magnesium carbonate, which was 



A 



Fig. I. 



employed as a source of carbon dioxide. ISTo stopcocks or rubber connec- 

 tions were used. In the experiment now being considered, some fflodea 

 canadensis was chloroformed for two hours, and then placed in A with 

 water and a little thymol (used as an antiseptic). The pump was worked 

 for some time after the last visible traces of gas had been removed, and 

 it is certain that no air remained in the tissues. The magnesium carbonate 

 was now heated, and the system was washed out twice with carbon dioxide. 

 Finally A was filled with carbon dioxide at about 2 cm. pressure, and was 

 sealed off at a with a small blowpipe flame. The Elodea was exposed to 

 light for 12 hours, and the tube was then attached to the pump again and 

 thoroughly exhausted, the gas being collected in a tube over mercury. A 



