A Alanometric Actinometer for the Visible Spectrum 77 



entering the vessel was about 2 cm. 2 , whereas the bottom area of the vessel was 

 about 9 cm 2 . The light beam was adjusted to the middle of the vessel. Thus, when 

 the vessel was shaken, no part of the beam passed outside the vessel. 



The total intensity of the light beam was measured bolometrically as described 

 elsewhere 5 . The bolometer had been calibrated against a Standard lamp of the 

 U. S. Bureau of Standards. The quartz window, which protected the Strips of the 

 bolometer, was 2 mm. thick and transmitted 82.5% of the radiation of the Stand- 

 ard lamp. 



The actinometer was shaken in the ordinary shaking-machine 3 - 1 with horizontal (not circular) 

 motion. Foaming never occurred. When equilibrium was reached in the thermostat in the dark at 

 20° C, the pressure was noted. The manometer was then taken out of the thermostat and the shak- 

 ing continued in air at about 20° C. Here the actinometer was illuminated with light of measured 

 intensity. After periods of 10 — 30 min., the illumination was interrupted, the manometer was again 

 placed in the thermostat and, after shaking 5 min. in the dark, the pressure was read. 



The purpose of this procedure was to avoid uncertain corrections for the light 

 transmission of the Windows of the thermostat and of the water in the thermostat. 

 The same mirror which reflected the light beam in a vertical direction into the 

 actinometer, by rotation could reflect the same light beam into the bolometer. If 

 / was the total intensity of the beam which reached the Strips of the bolometer, 

 1.04 • /was the total intensity which entered the actinometer. Thus, the corrections 

 for the different paths of the light were reduced to a minimum. 



We have tried to determine whether shaking is necessary during illumination, 

 and always found distinctly smaller quantum yields when resting vessels were 

 illuminated. The layer of the Solution in which the main part of the light is ab- 

 sorbed is very thin and it seems that here, in the resting vessels, concentrations 

 decrease to such a degree that light energy is lost. 



Here follows an example of determination of the quantum yield: 



20°C; 100% 2 ; 585— 615 m// (600 m//). 



V = 14.10 cm. 3 v F = 5 cc. v G = 9.10 cc. K . 2 = 0.893. 



200 mg. thiourea. 2 mg. ethyl-chlorophyllid, dissolved in pyridine. The solubility of O2 at 20° C. 



in the actinometer Solution was determined manometrically. a = 0.092 was found and used for 



273 



vg \- vf ■ a 



T 

 the calculation of the vessel-constant Kq 2 = - - (mm. 2 ). The total light intensity 



entering the vessel was / = 0.109 micromoles of quanta/min. The manometer readings were: 



20 min. dark 



15 min. light 5 min. dark — 39 mm. 



34 8 



= — 34.8 mm. 3 = — = 1.55 micromoles Oo. 



22.4 



1-55 1.55 nnMB 



w = = = 0.945. 



15 x 0.109 1.64 



In the following table are presented the results of a continuous series of experi- 

 ments, the time of illumination being 30 min. in the blue or 15 min. in the other 

 spectral regions. In each of the four spectral regions mentioned in the table, the 

 light was tested for unabsorbed light, especially for infrared. Between the exit slit 

 of the monochromator and the bolometer we placed an absorption cell of 1 cm. 

 thickness. When the cell contained acetone or pyridine, it transmitted 91% of the 



