QUANTUM YIELD MEASUREMENTS BY THE MANOMETRIC METHOD 1105 



l)ractically instantaneous. No correction (of the type illustrated in fig. 

 29.1) was therefore used to account for diffusion through the liciuid and 

 the exchange between the two phases. Instead, the yields were now cal- 

 culated from manometer readings made at the very moment of changing 

 from darkness to light, or from light to darkness. Errors caused by 

 "physical lag" had been considered of prime importance in 1923 and 1948; 

 in some examples given in these earlier papers the calculated quantum 

 yields would have been quite different without correction for this lag. 



Time Schedule. The two vessels were filled simultaneously with ali- 

 quots of the same culture, and exposed alternatively to the same beam 

 of light (e.g., 10 min. light on vessel I, then 10 min. light on vessel II, 

 then again 10 min. light on vessel I, and so on). Both vessels (total 

 volumes 14 and 18 cc, respectively) contained the same amount of liquid 

 (7 cc). It was argued that whatever physiological differences may have 

 existed between the cells in the two vessels during the first exposure 

 (because of a "phase difference" of 10 min.), must have disappeared after 

 several light-dark cycles. This is plausible; however, Emerson and co- 

 workers found that at least five or six (10 min. light + 10 min. dark) 

 cycles may be needed to eliminate the initial difference, while in many of 

 Warburg and Burk's published experiments (cf. table 29. IV) only 2 or 3 

 cycles were used. The alternate exposure schedule was altogether aban- 

 doned in almost one half of all experiments — namely those in which 

 "background" illumination was used to compensate respiration. 



Light Measurement. No physical determination of light intensity 

 was made by Warburg and Burk. (Bolometers had been used in earlier 

 experiments, both by Warburg and by Emerson.) Instead, light intensity 

 was determined by means of the ethyl chlorophyllide - thiourea actinom- 

 eter, for which a quantum yield of 1.0 was previously found (bolo- 

 metrically) by Warburg and Schocken in Emerson's laboratory (cf. 

 chap. 35). The quantum yield of the actinometer is known to decline 

 with increasing light flux, particularly >0.1 /zeinstein/min. Many runs of 

 Warburg and Burk were carried out in stronger light; the intensity of 

 the beam was reduced in these experiments to about 0.1 ^einstein/min. 

 by means of calibrated wire screens before it was directed on the actinom- 

 eter. 



Light Intensity. In Warl)urg's 1923 and 1940 measurements, the 

 use of very weak incident, light was considered important, since the 

 quantum yield was found to decline significantly (cf. fig. 29.8) with in- 

 creasing light intensity, beginning as early as at 1000 erg/cm. ^ sec. (about 

 0.03 )ueinstein/cm.2 min.). In the Warburg-Burk work, much higher 

 incident light intensities werc^ uschI: instead of uniform illumination (jf 

 almosi the whole boltoin aiea, as used in earlier experiments (1923, 1948), 



