68 PLANT RESPIRATION 



the contents of the flask from the external air. The solution 

 which has been thus introduced has only a pale brownish color 

 after months of work. 



To insure a uniform generation of hydrogen, attention must 

 be paid to make sure that the acid does not become too dilute 

 through the formation of zinc sulphate. As soon as the acid 

 begins to rise in the bell jar so that the sticks of zinc are partly 

 immersed in the solution, the lower layer of liquid, consisting of 

 heavier zinc sulphate solution, must be removed and fresh 

 sulphuric acid added. This can be easily accomplished while 

 the current of gas continues to flow without interrupting the 

 experiment. In the case of accurate and refined experiments 

 the acid must touch only the lower ends of the zinc sticks even 

 for a vigorous stream of hydrogen. An apparatus fitted with 

 sufficient zinc can be used a long time in this manner and yield 

 very large amounts of hydrogen. If there are traces of arsenic 

 in the zinc or in the acid the gas is passed first through a solution 

 of iochne, then through concentrated potassium iodide solution, 

 and finally through an alkaline solution of pyrogallol. 



The gas can also be drawn from a cylinder (Bombe). The 

 compressed hydrogen is generally prepared by electrolysis and 

 is therefore entirely free from the trihydride of arsenic, though 

 it often contains some oxygen. In the use of compressed hydro- 

 gen the stream of gas must be passed through at least two wash 

 bottles of alkaline pyrogallol solution and of course in the form 

 of separate bubbles. A considerable velocity of the gas current 

 is therefore prohibited in this case. 



Although most of the investigations of anaerobic respiration 

 have been carried out by the use of hydrogen, it appears ques- 

 tionable whether hydrogen actually represents an entirely inert 

 gas. We now know that most of the oxidations and reductions 

 in plants are to be traced to the sphtting-oft" or the adding-on 

 of hydrogen. Of course molecular hydrogen is nearly inactive, 

 but it is a fact that for plants it constitutes an absolutely un- 

 usual gas medium. Hence the use of nitrogen is always more 

 commendable and refined. 



For a continual passage of gas it is best to use compressed 

 nitrogen which is drawn from a cylinder with a reducing valve. 



