PHOTOSYNTHESIS 



109 



stopper and tube are lifted from the chamber and placed upright in the 

 pneumatic trough, deeply enough to allow the stopper to be taken off with- 

 out the admission of air to the tube. The zero mark of the tube is then 

 brought exactly to the water surface; the upper stop- 

 cock is cautiously opened, permitting the water to rise 

 slowly to the zero rrark, when the stop-cock is again 

 closed, shutting into the tube exactly 10 cc. of the gas 

 to be analyzed. First the quantity of carbon dioxide in 

 the tube is determined, which is accomplished by aid 

 of the reagent tube described on page 105. Next a 

 determination of the percentage of oxygen present is made 

 by another reagent tube, containing pyrogallate of potash, 

 used in the same manner. Some slender vessel is then 

 slipped under the measuring-tube, which is removed and 

 supported for observation, as shown in the figure. 

 Corrections are to be treated as described earlier (page 

 105). The measuring-tube should always be washed 

 quite free from the reagents at the close of every analysis, 

 and the ground joints should be kept slightly lubricated 

 by the usual wax. 



In studying the process with beginners, the demon- 

 stration is the more striking and conclusive to them if 

 a second instrument is set up like (and beside) the first, 

 but covered completely from light; while even a third, 

 like these two except that it has no plant, may advan- 

 tageously be added. When interpreting the final results 

 it must te remembered that the experiment is started 

 with only 90% (if 10% carbon dioxide is used) of air, 

 that is, 72% of nitrogen and 18% of oxygen. 



It is quite possible to adapt a photosynthometer from 

 a graduated tube, the bulb of a calcium-chloride tube, 

 and a rubber stopper, as shown by the accompanying 

 figure (Fig. 27), the principle of its use being nearly 

 identical with that of the instrument just described. Its 

 capacity must of course he accurately determined; the 

 proper quantity of carbon dioxide is put into the 

 graduated tube, which is sealed with the finger until 

 pushed into the bulb, the small opening of which is 

 immediately afterwards sealed with tire tape. 



Fig. 27. — Simple 

 photosynthom- 

 eter; X$. 



Particulars in text. 



In connection with the use of a 10% carbon- 

 dioxide mixture in the preceding experiment, the 

 student should look into the current statements 

 as to the strengths of that gas green plants can endure, and 

 also into the supposed optimum strength, upon which latter point 



