6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 87 



Measurements of the light intensity were also made with a photronic 

 cell in order to express the results in foot-candles. The photronic cell, 

 with a dark shade heat-absorbing filter, giving a resulting sensitivity 

 curve approximating the visibility curve, was calibrated against a 

 standard of illumination. Readings of the photronic cell, at any 

 particular light intensity, were taken at various vertical positions 

 along the growth chamber and at various orientations. A mean of 

 these readings is taken as the mean intensity on the leaf surface. In 

 order to convert the value given in arbitrary units to value in foot- 

 candles, multiply by 4.96. The above limits of intensities expressed in 

 foot-candles are 947 and 80, respectively. 



It is interesting to compare the light intensities used in this experi- 

 ment with the total solar intensity. The daily values of the total solar 

 radiation received on a flat surface, expressed in calories per cm'-, are 

 given in the Monthly Weather Reviezv. On cloudless days in Washing- 

 ton during May and June the total solar radiation may be as high as 

 600 or 700 calories per cm-. On the basis of a lo-hour day we may 

 conclude that the average solar intensity for cloudless days during 

 these months is approximately one calorie per cm- per minute. The 

 intensities used in this ex}>eriment varied from 0.021 to 0.246 calorie 

 per cm- per minute. Thus the intensities used varied from 1/48 to ^ 

 that of sunlight. 



APPARATUS FOR MEASURING CARBON DIOXIDE 



The apparatus ' for determining the carbon dioxide content of the 

 air, shown in Figure 2, is one developed and built at the Fixed Ni- 

 trogen Research Laboratory, United States Department of Agricul- 

 ture, and loaned to us for this work. The principle is to scrub a 

 definite volume of the air with a definite volume of potassium hydrox- 

 ide solution and to determine the titer of the resulting solution by 

 means of its electrical conductivity. 



The flow of the KOH solution (in this case .02 N) from the large 

 reservoir, R, is maintained fairly constant by means of the constant 

 level device, S, so that the liquid falls from the tip of the capillary 

 tube. A, in drops of uniform volume at intervals of about eight sec- 

 onds. Each drop falls through the bulb, B, upon the opening, C, 

 trapping the air in the tube, CD, and forcing it along to the constric- 

 tion in the tube at E. The constriction in the tube is of such size and 

 shape as to arrest the drop at this point until displaced by the impact 



^ White, Ernest C, An apparatus for continuous gas analysis. Journ. Amer. 

 Chem. Soc, vol. 50, pp. 2148-J154, 1928. 



