Absorption of Edntgens Eays by Solutions of Metallic Salts. 415 



plate was placed behind the lead sheet, and exposed generally for ten 

 minutes to the rays. The tube was placed in such a position that the 

 anti-cathode was in the normal drawn to the photographic plate from 

 the point midway between the two holes, and was 12 inches distant 

 from it. The plates, after development, were placed in the photometer, 

 and the relative intensities of the two spots compared, the arrangement 

 adopted being that shown below, fig. 1, in which the two spots were 

 illuminated by the light from two candles. 



Fig. 1. 



BLack screen to 

 keep off direct light* 



Cduic/Le Photographic 

 fixed at *o cms. ptate 



Ground glass 



225 200 17 ft 150 125 100 J 75 50 



i ■ . ■ . i ■ ■ ■ ■ i ■ ■ . i ■ ■ . ■ i . . 



25 



Scdle in cms 



A ground glass screen was placed behind the plate to obtain uniform 

 illumination of the spots. It was found with this arrangement that 

 the relative intensities could be determined with considerable accuracy, 

 but that the relative intensities depended to a certain extent on the 

 time of development, as has also been noticed by Hurter and Driffield. 

 Another disadvantage of this method was that the allowance that had 

 to be made for the absorption of the glass was a comparatively large 

 one with many solutions, and that the error incident on the determina- 

 tion of . this correction was such as to materially alter the relative 

 absorbing powers. Thus if a be the percentage of absorption due to 

 the glass of the cells (which was within limits of experimental error 

 equal for the two cells employed), b that due to the water in one cell, 

 and b + c that due to the solution in the other cell, c being supposed to 

 be that part of the absorption due to the presence of the dissolved salt, 

 the ratio actually observed in this method by comparing the intensities 

 of the two spots is — 



a + b + c _ absorption due to the cell and dissolved salt 

 a + b absorption due to the cell and water 



If therefore c is small compared with b or a, the accuracy attainable is 

 very much reduced. For very absorbent substances, however, this 

 method gave fairly good results. 



A very important question in connection with this investigation was 

 the standard with which all these aqueous solutions were to be com- 



