38 GAS ANALYSIS. [CH. II 



It is by no means necessary to employ a tube of the 

 above described form. We often employ tubes of test-tube 

 form of 2 cm. internal diameter and containing 100 c.c. 

 Oleander leaves (which are especially good material in 

 the winter months) fit these tubes well. The mercury is 

 raised to the desired height by a thick- walled india-rubber 

 tube pushed up , into the cavity, and connected with a 

 water air-pump. The rubber tube is then closed between 

 the fingers and drawn out : if in this process a few drops 

 of mercury are drawn into the tube, they may be sucked 

 (by turning on the pump) into a bottle fitted like a 

 washing bottle, which serves as a trap between the pump 

 and any vessel to which suction is to be applied. 



(52) WinklerSempel apparatus. 



For demonstration purposes, where it is desirable to 

 avoid barometer readings, calculations, &c., fair results may 

 be obtained with the Winkler-Hempel apparatus. 



A jar, J, fig. 7, containing leaves is filled with air con- 

 taining about 8 7o of CO2: the exact proportion is of no 

 importance, but it must be accurately determined at the 

 beginning of the experiment. The bent tube t serves to 

 draw off a sample of the gas in the jar J, and as it is 

 drawn off, the water flows through the tube I fi-om the 

 beaker outside, into the second vessel inside i. The 

 tubes t and I are now clamped, and the apparatus exposed 

 to bright light for 4 or 5 hours when a fresh sample of gas 

 is drawn off and analysed. The water introduced absorbs 

 some of the CO2 and causes an error, which however is not 

 so serious as to interfere with the results for demonstra- 



