6 RESPIRATION. [CH. I 



standard decinormal hydrochloric acid, using phenol- 

 phthalein (colourless with acids, pink with alkalis) as 

 an indicator. Compare 20 c.c. of the original baryta 

 solution with the same acid. The difference between the 

 amounts of HCl required to neutralize the two samples 

 gives a measure of the amount of baryta that has been 

 removed from each 20 c.c. of the liquid by combining 

 with carbon dioxide. The whole amount of CO2 produced 

 in the experiment is of course 2^ times as great, and it 

 may be readily estimated from the following data: 



1 c.c. ^ HCl = 0-0022 gr. CO2 = 1'19 c.c. COo at 15° C. 



(4) Winkler- Hempel apparatus. 



By this apparatus the volume of CO2 given off by 

 respiration in a kno\Mi time may be fairly well deter- 

 mined. 



To get a good result it is well to use a considerable 

 quantity of material, say 200 peas just beginning to 

 germinate. They are placed in a conical flask of 400 c.c. 

 or 500 c.c. capacity and closed by a good rubber cork. 

 After 1 or 2 hours a sample of gas is drawn off for analysis. 

 This may be managed by an arrangement similar to that 

 shown in fig. 3, in which water flows from to i as the 

 gas is withdrawn. The arrangement figured was meant 

 for experiments on assimilation when several determina- 

 tions of the CO2 in the jar J are made : in the present 

 experiment the test-tube i is omitted and the water flows 

 into the bottom of the flask. Thus only a single sample 

 of gas can be analysed with accuracy since the water 



