242 PHYSIOLOGICAL CHEMISTRY. 



The relations 



E E^ E'^_ 

 C' 7' ~C"' 



etc., must be all equal and constant for the same substance. This con- 

 stant ratio is a characteristic which connects the light-absorbing power 

 of a solution with its strength ; it may be represented by the letter A and 

 be termed the absorption ratio. Hence, for a given color 



The value of the constant A must be found for a given spectrum region 

 by employing a series of suitable concentrations. The determination of E 

 consists in finding the value of the reduced light as compared with the 

 original ; from the formula given above the extinction coefficient is equal to 

 the negative logarithm of this diminished intensity. In the various forms 

 of photometers employed in this kind of work the peculiar measuring 

 mechanism permits the direct and simple estimation of the intensity of the 

 light after absorption as compared with the light before absorption. We 

 find /' then as a fraction, and E is the negative logarithm of this. For 

 example, suppose we have in a liter 0.25 gm. of an absorbing substance. 

 The concentration, or C, is 0.00025, which represents the value per cubic 

 centimeter, taken as the unit of volume. Next, suppose we find with our 

 special measuring instrument that the value of the light after absorption is 

 only 0.0436 of the original, that is about ^. Substituting in our formula 

 we have 



E log /' = log 0.0436 = 1.36051 

 and finally 



C 0.00025 



In this way, by repeating the observations with a number of different 

 strengths of solution of the substance, we find the value of the constant. 

 As the individual observations may differ a little the mean must be taken. 

 A becomes thus fixed once for all for a given spectral region, and its value 

 may be employed to determine the concentration of unknown solutions since 



C EA. 



Quantitative spectrum analysis by absorption is based on these very 

 simple principles. Blood or other substance to be examined is placed in a 

 cell with plane parallel sides, preferably exactly I cm. apart. The cell 

 should be half filled and is brought into proper position in front of a 

 spectroscope with a double slit, the level of the liquid just reaching to the 

 top of the lower slit. The light from the illuminating lamp enters the 

 upper slit directly and the lower one through the colored liquid. If the 

 two slit openings were the same to begin with the upper one must now be 

 narrowed until the light passing through it is reduced to the intensity of 

 the light through the blood and the lower slit. The width of each slit 

 may be measured on a micrometer screw head or in some other convenient 



