2i 4 HEMOGLOBIN. 



1. Relation between the concentration of a solution and the percentage 

 of light absorbed by it. Before investigating the theory of the methods 

 of spectrophotometry, to be subsequently described, it is essential to 

 examine (1) the relation which exists between the power of light 

 absorption exerted by a coloured liquid of constant composition and the 

 thickness of the layer traversed ; (2) to study the influence of concen- 

 tration on the absorption of light by a stratum of a liquid holding a 

 colouring matter in solution. 



It was shown by Lambert that if light of intensity /, by transmission 

 through one layer of an absorbing medium of thickness 1, has its in- 



tensity reduced to 7- =. - by transmission through d such layers, the 



T?/ 7Z/ 



final intensity of the light, which we Shall represent by /', will be re- 

 duced to , i.e. I' = . Beer showed that Lambert's law holds good, 



not only for transparent solid media, but also for liquids, i.e. that the 

 amount of light absorbed by a solution of a given colouring matter of con- 

 stant concentration is dependent upon the thickness of the stratum?- This 

 law is only true, however, in respect to monochromatic light. 



We must now examine the influence of the concentration of a liquid 

 containing a colouring matter in solution upon the percentage of light 

 which it absorbs and transmits, when the stratum examined remains of a 

 constant width, 1. It has been experimentally proved that the absorp- 

 tion exerted by a stratum of a coloured solution of known width is 

 equal to that exerted by a stratum twice as thick of a solution of half 

 the concentration ; i.e. the absorption which light undergoes in passing 

 through a stratum of coloured liquid of unit thickness increases propor- 

 tionally to the concentration. 



2. Definition of the " extinction-coefficient." In their photo-chemical 

 researches, studying the comparative absorption of light by different 

 gases, Bunsen and Roscoe' 2 introduced the conception of, and defined, 

 the so-called extinction-coefficient. They ascertained the relative thick- 

 nesses of the strata of various media required to reduce the intensity 

 of light passed through them to one-tenth of its initial value, and defined 

 the extinction-coefficient as the reciprocal of the number expressing the 

 width of the stratum of a given medium., required to reduce the intensity of 

 light passed through it to one-tenth its initial value. 



For any given coloured medium, e.g. a solution of a colouring matter 

 of a definite strength, there must be a definite thickness of layer which 

 we shall call d, capable of reducing the intensity of light to one-tenth 



its original value. The reciprocal of d is -v, and if by e we represent 

 the extinction-coefficient, 



As will be shown in the sequel, the method of spectrophotometry 

 discovered by Vierordt rests upon the determination of this constant g, 

 for particular, very limited, regions of the spectrum. The practical diffi- 

 culties of varying the thickness of the stratum of a coloured liquid, until 



1 When the thicknesses of various strata increase in arithmetical, the intensities of 

 the light decrease in geometrical, ratio. 



2 Ann. d. Chem., Leipzig, 1857, Bd. ci. S. 238. 



