A CHEMICAL PHOTOMETER 235 



(In most of his ei:periinents Dr. Bacon used a proportion of 1 

 to 10. In the present tests, however, the amount of oxaHc acid 

 was increased in order that long exposures might be made.) 

 The solutions were brought together in Florence flasks of 100 cc. 

 capacity, plugged with loose wads of cotton, ^ and the flasks so 

 adjusted in holes in a blackened boaM that the surface of the 

 liquid in each flask was flush with the upper surface of the board, 

 the neck of the flask being inclined to the north so that no 

 shadow would be cast upon the liquid. This method of exclud- 

 ing the light from the sides of the flask was adopted in order 

 that only the horizontal surface of the liquid would be exposed 

 to the sunlight and thereby be more nearlj^ comparable with 

 the horizontally exposed receiving portion of the pyrheliometer. 

 After exposure the oxalic-acid-uranium-acetate mixture was 

 titrated with potassium permanganate (2N solution) in the 

 following manner: the mixture was increased to a convenient 

 volume for titration, either in the original flask or after having 

 been transferred to a beaker, by the addition of distilled water. 

 The volume to be titrated was then heated to 70°C., made 

 strongly acid by the addition of 1:1 sulphuric acid and the 

 potassium permanganate end-point determined while the solu- 

 tion was still hot. Though Dr. Bacon states that he removed 

 the uranium salt before titration by the addition of a slight 

 excess of ammonium hydrate, the writer found that aliquots of 

 the same solution gave no differences in the amount of oxalic 

 acid present, whether the uranium was removed or not, and, 

 therefore, did not use the ammonium hydrate. It was found 

 that in cool weather a 10% solution of oxalic acid became super- 

 saturated, when the temperature dropped during the night, to 

 such an extent that the stock bottle contained crystals the 

 following morning when it was desired to make up fresh mix- 

 tures for exposure. Because of this fact it was necessary to use 

 double the quantity of a 5% oxalic acid solution. 



-The flasks could not be completely closed. since Dr. Bacon says, "For all 

 practical purposes the decomposition of oxalic acid under the influence of uranyl 

 salts may be assumed to take place as follows: H2C2O4 — > CO + CO2 + H2O." 

 Some outlet, therefore, was necessary for the escape of the gases generated. 



