The Intensity of Transmitted Light. 153 



than unity. At any instant the intensity of the transmitted 

 light will be 



I = I^e-^-"^. (3) 



Substituting from (2) this becomes 



As the composition of the medium is undergoing gradual 

 change, q must be some function of the time, let this be 

 ^ {t\ then the last equation may be written 



I = I,6-'"^e^^^'^> (4) 



The constant c may be either positive or negative, for 

 the transparency of the solution may either diminish or 

 increase ; if c have the value 0^ then a change in chemical 

 composition will have no influence on the nature of the 

 transmitted light. If q be given as a function of the time, 

 then by means of the above formula the intensity of the 

 transmitted light may be determined, and conversely by 

 observations of the intensity we may determine at any time 

 the quantity of B existing in the solution. 



There is another enquiry which seems to have more 

 mathematical interest than the preceding, and which was 

 suggested to me some years since when engaged in 

 making experiments with various coloured solutions in 

 order to establish some simple rule for determining 

 quantities by colorimetry, I had occasion to try a solution 

 of potassium ferricyanide, but the action of light on it was 

 50 decided, that in a short time, a rule which was satisfactory 

 in some other cases could not be applied to determine the 

 quantity of salt present. The action of light in this and 

 other instances which might be adduced is not of a sudden 

 character, as in the case of the explosion of a mixture of 

 hydrogen and chlorine, but gradual and continuous ; when 

 placed in the dark the action is reversed, and the fluid 

 nearly regains its original appearance. What becomes of 



