RESPIRATION. 



97 



ic acid in the tube, iv/iieh is easily effected by forcing the end 

 (>/' the tube do-vn into the l>o.v of the odd. Care must be taken that 

 the rubber stoppers are air-tight : old stoppers are very likely to leak 

 air ti'en u'hen seemingly tig/it. I 



I" tubes are recommended for this experiment because they are easy 

 to obtain, and their shape adapts them well for the purpose. Straight 



Fr<;. 22. ARRANGEMENT FOR DETERMINING THE GAS-EXCHANGE IN RESPIRATION. 



One-third the true size. 



tubes with wire netting plugs to hold the seeds from falling can be used 

 as shown in Fig. 21, on the left, but unless the oats are put in with great 

 care, and all surplus water is removed from them, the water will run down 

 and the potash diffuse up the streams and kill the seeds. In this respect 

 the U tubes have a great advantage. Best of all, however, though rather 

 expensive to supply in quantity, are the ijo-cc. "absorption-tubes" 



