MEASURING PHOTOSYNTHETIC ACTIVITY 237 



marily interested in the influence of plants on air. It was recognized 

 early that no single chemical element enters so intimately into the exist- 

 ence of man on the earth as does oxygen. The manifold phases of this 

 fact have led to the investigation of the relations of oxygen in many 

 branches of science, in technology and the arts. After the discovery of 

 oxygen by Priestley and Scheele the question of the composition of the 

 atmosphere became a popular subject with scientists and the scientific 

 dilettantes. From then on a variety of methods was devised and their 

 virtues and faults widely discussed. The development of the methods 

 of air analysis forms a most interesting chapter in the history of chem- 

 istry which has been admirably compiled by Kreusler -^ and more re- 

 cently by Benedict.-^ Both of these works are rich sources of information 

 on physiological gas analysis. 



With the development of more rapid and accurate methods of air 

 analysis there has been a very general trend away from gravimetric 

 methods toward volumetric methods of oxygen determination, so that at 

 the present time only the latter principle demands consideration. For 

 practical and accurate determinations of oxygen in work on respiration 

 and photosynthesis it is desirable that these can be carried out without 

 consuming too much time and that successive determinations can be 

 made easily. It is essential to have an efficient absorbent for oxygen 

 which means that: (1) the oxygen should be completely absorbed; (2) in 

 the chemical reaction between the oxygen and the absorbent no gases 

 should be given ofT into the residual gas and thereby increase its volume ; 

 (3) any temperature changes produced by the absorption must be com- 

 pletely and readily compensated. Besides these requirements it is, of 

 course, essential that the oxygen absorbent does not afifect any other 

 gas in the gas mixture, i.e. that the contraction in volume as measured 

 should be due only to the absorption of oxygen. Finally, as is general 

 in gas analysis, changes in barometric pressure taking place during an 

 analysis must be fully compensated and the tension of aqueous vapor in 

 the gas must be the same in the original and in the final measurement 

 after absorbing the oxygen. 



A very large number of oxygen absorbents have been suggested and 

 used in various apparatus. Those which demand consideration for our 

 purpose are briefly discussed below. 



The use of a heated copjoer spiral for the absorption of oxygen was 

 introduced by von Jolly -- and has been extensively used. Kreusler im- 

 proved the method so that it is now one of the most exact methods 

 known.^^ 



""Kreusler, U., Laniw. /a/irfc., 14, 305 (1885). 



" Benedict, F. G., "The Composition of the Atmosphere, with Special Reference 

 to Its Oxygen Content," Carnegie Institution of Washington, Pub. No. 166 (1912). 



"Jolly, Ph. von. Wiedemann's Ann. N. F., 6, 520 (1879). Kreusler, U., La>uiw. 

 Jahrb., 14, 305 (1885). 



" Durig, A., Biochem. Zcit., 4. 65 (1907). For very accurate work, it must be 

 remembered that copper oxide loses oxygen at high temperatures ; cf., Gmelin- 



