A PROMISING CHEMICAL PHOTOMETER FOR PLANT 

 PHYSIOLOGICAL RESEARCH* 



CHARLES S. RIDGWAY 

 Bureau of Plant Industry, U. S. Department of Agriculture 



Botanical literature is replete with references regarding the 

 effect of light upon plants from the viewpoint of the physiologist, 

 the anatomist, the histologist, the ecologist and the agriculturist. 

 Numerous methods and instruments have been devised and 

 used, and with some success, for the determination of the inten- 

 sity or nature, or both, of the insolation of the plant under 

 observation. So far as the writer is aware, all of these methods 

 and instruments are possessed of objectionable limitations, either 

 in reliability, initial cost, or cost of operation. It seems advis- 

 able, therefore, to call attention to a photometer which, at the 

 present stage in its trial, at least indicates its practicability in 

 connection with investigations of the light relations of plants. 

 Allusion is had to the use of oxalic acid and uranium salts as 

 advocated by Dr. Raymond F. Bacon. ^ 



Through the courtesy of the Weather Bureau of the United 

 States Department of Agriculture and with the kind cooperation 

 of Prof. H. H. Kimball, of that Bureau, some comparisons have 

 been made of the records obtained by the Callender pyrheliom- 

 eter with the results of exposures of .the chemical photometer, 

 with the idea of standardizing the latter. 



In testing out the chemical photometer, the writer used 

 uranium acetate and oxalic acid in the proportions of one part 

 by weight of the former to twenty of the latter; that is, 5 cc. 

 of a 1% (0.023 molecular) aqueous solution of the uranium salt 

 to 20 cc. of a 5% (0.71 molecular) aqueous solution of the acid. 



* Published also in The Monthly Weather Review, March, 1918, 46: 117-119. 

 ^ Bacon, R. F., A solution of oxalic acid and uranium salts as a chemical 

 photometer. Philip. Jour. Sci. A; 5: 281-303. 1910. 



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