METABOLIC PROCESSES IN THE PLANT. 265 



and it can be very conveniently and rapidly filled before each 

 experiment by connecting the off-flow tube (at the point i) by 

 means of rubber tubing with the water supply. It is also 

 advisable to use as aspirators very large vessels, e.g. large acid 

 carboys. 



The Hydrogen is prepared in the large Kipp's apparatus TF 

 from chemically pure, Arsenic-free zinc, and Hydrochloric acid 

 free from Arsenic. In order to make sure of removing every trace 

 of injurious admixtures, the gas is passed through the wash-bottle 

 Ue, which contains a solution of Potassium permanganate, and 

 through the U-tube S, containing fragments of pumice soaked 

 with a solution of Silver nitrate. The contents of both these 

 vessels must be frequently renewed. The Hydrogen streaming 

 through the absorption column K f , containing concentrated solu- 

 tion of pobash below and fragments of caustic potash above, and 

 through the U -tubes K" and 7 7// , containing fragments of pumice 

 soaked with potash solution, is deprived of every trace of Carbon 

 dioxide. Further, we may insert between K" 1 and the worm of 

 the respiratory vessel R, a small bottle containing a little concen- 

 trated Sulphuric acid, to serve as a stop valve. This valve, made 

 like the valve Sch, but not represented in the figure, renders 

 absolutely impossible any backflow of Carbon dioxide from the 

 spiral, which however, even without this precaution, would hardly 

 take place. 



The vessel intended for the reception of the research material 

 may have, according to requirements, a capacity of from 200 to 

 400 c.c., and is continued below into a spiral tube which ascends 

 alongside the respiratory chamber proper, It. This last is closed 

 at the top with a rubber stopper, through which pass a ther- 

 mometer, T", and an obliquely bent glass tube. The thermometer., 

 graduated to tenths of a degree, should have a long cylindrical 

 bulb, which is placed in the midst of the research material. The 

 zero of the thermometer scale must appear above the rubber 

 stopper, so as to render it possible to read clearly when working 

 with low temperatures. At the bottom of the respiratory chamber 

 we place some wet glass wool, and dn this the plant material, 

 perhaps moistened. 



The respiratory vessel is placed in an earthenware vessel, G, filled 

 with water, provided with a wooden cover, and supported by the 

 tripod D. By adding cold or warm water, or by heating with a 

 gas flame, we can regulate the temperature of the water, as indi- 



