236 CHAELES S. RIDGWAY 



A series of thirteen comparisons of the chemical photometer 

 with the pyrhehometer was run during the months of May and 

 June, 1917. The length of exposure of the solution ranged 

 from eight to twenty-four hours, although in most cases the 

 flasks were first exposed after dark in the evening and taken in 

 after dark the next evening, giving an exposure of a full twenty- 

 four hours. The ratio of the number of calories recorded by 

 the pyrheliometer to the number of grams of oxalic acid decom- 

 posed, for each exposure, the mean ratio of all the exposures, and 

 the percentage departure of each from the mean were computed. 

 It was found that, with a mean of 978 for all the exposures, in 

 nine cases the deviation from the mean ranged between 0.2 

 and 3% on either side, and that of the four other exposures, one 

 was 6% below the mean and three were 5, 7 and 8% above, 

 respectively. 



Tests to determine the stability of the oxalic-acid-uranium- 

 salt mixture in the dark, both before and after exposure, were 

 raade repeatedly and indicated that no appreciable decomposi- 

 tion of the oxalic acid takes place within three days if the mix- 

 ture is kept in a dark closet. The effect of the diffuse light of 

 the laboratory was also tested. The results showed that it 

 would be necessary to expose the photometer for several hours 

 in order to effect a measurable decomposition of oxalic acid 

 when the proportions mentioned above are used. The tempera- 

 ture coefficient of the reaction was disregarded, since Dr. Bacon 

 states that for a remarkably wide range of temperatures this 

 factor need not be considered. 



One application of the chemical photometer was demon- 

 strated, at least to the satisfaction of the writer, in determining 

 with it the transmission coefficient of a piece of tobacco shade 

 cloth of 12 by 12 mesh in connection with investigations of the 

 light relations of the tobacco plant. A flask containing the mix- 

 ture was exposed (from 9.30 a.m. to 2.30 p.m. on March 21, 1917, 

 a cloudless, bright day) to the sunlight which passed through 

 the cloth stretched over the South side and top of a small 

 frame. During the same period a second flask was exposed to 

 uninterrupted sunlight and a third to the light of a portion of 



