6 ON THE ABSORPTION-FORMULA IN PHOTOMETRY. 



stronger than the other there was a slight departure from 

 the simple rule of colorimetry which held in other cases ; 

 this I thought might be due to the absorption of the water 

 employed. The probability that this is the cause would be 

 increased if the value of p deduced from one experiment, 

 being applied as a correction to the other experiment, gave 

 consistent results. 



If we write p in the form k p , the formula to be used may 

 be written (M 9 )*=(W)/'. This leads to the relation 



{q+p)l=tf+p)l. 



The standard solution contained i*2 cub. c. of a strong 

 carbon diffusion in 500 cub. c. of water ; and the length of 

 column was 21*2. Comparing with this a solution con- 

 taining 9/6 cub. c. in 500 cub. c. of water, on one occasion 

 I made 2^94 the equivalent column, on another occasion 

 2*87, and on a third occasion 2*8. The mean of these 

 three results is 2*87. Hence the corrected formula will be 



(Q + o-ii4)Z=(Q' + o-ii4)/', 



Q, and Q,' denoting the number of cubic centimetres of the 

 strong diffusion added to each cylinder. In the following 

 table I have recalculated the results given on page 197, 

 vol. xix. of the ' Proceedings': — 



A. 



B. 



C. 



2'02 



1*92 



177 



i-59 



1-44 



1-32 



I '22 



115 



ro6 



I-0 5 



0*96 



o-88 



C89 



0-82 



076 



O78 



072 



o-66 



A denotes length of column by experiment, B denotes 

 theoretical length calculated by corrected formula, C the 

 theoretical length deduced from uncorrected formula. It 

 will be noticed that the discrepancies between A and B are 

 less than those between A and C. 



