416 Lord Blythswood and Dr. E. W. Marchant. 



pared. It seemed most satisfactory, since all the solutions that were 

 tested were aqueous, to choose water for the standard absorber, and all 

 numbers given are relatively to water. 



Now, to determine the relative absorbing power of the salt in solu- 

 tion, compared with the absorbing power of water, we may consider 

 the absorption of the solution to be split up into two parts, one due to 

 the absorption of the water of which the solution is made up, and the 

 other part due to the salt dissolved. 



Now, as above, let a = percentage absorption due to cell, b = per- 

 centage absorption due to the water, c = percentage absorption due to 

 the salt. Then first comparing the intensities of two spots, as above, 

 one obtained through an empty cell and one through a cell filled with 

 water, we have 



■ a + b Tr , . ■, b Tr . b 



— = Ki (say), 1 + - = Ki, and a = ^— — . 



Then, comparing the intensities of two spots, one obtained through a 

 cell filled with water, and the other through a cell filled with the 

 solution, we have 



a + b + c jr / x 

 = K 2 (say), 



c = (« + *>) (K 2 -l), 

 and substituting for a, we have 



_ ,K,(K 2 -.l) 



e _K L (K i -l) 

 b ~ (K, -1) - 



The assumption here made is that the intensity of the black spot on 

 the negatiye is proportional to the intensity of the light falling on it. 

 With the cells used Ki = 1*7. 



The second method used for the determination of the relative 

 absorbing powers, was based on the fact that the amount of absorption 

 of any substance varies with the thickness of the layer traversed by 

 the ray. 



The arrangement of the apparatus was as shown in fig. 2. 



EE is a wooden base supporting the two uprights HH, to which is 

 attached a thick lead sheet (shaded diagonally in the figure). In the 

 uprights, HH, three grooves are cut, into which fit the bases of the 

 three cells. 1, 2, 3, which are thus supported one above the other on 

 their wooden stand. The cells 1 and 3 are wedge-shaped, being 

 137 mm. long, and 20 mm. wide at the broadest part, sloping away to 

 nothing at the thin end of the wedge. The cells are about 25 mm. 



