some Copper Salts in Aqueous Solution. 329 



Further, according to Beer's law, if there are two solutions 

 of the same substance, of concentrations c and d, and in 

 lengths of layer I and V } such that 



d=di', (1) 



they will absorb the same fraction of the light which passes 

 through them. 



Suppose they both transmit the fraction F, then 



^-i^ande-- 1 ^, 



(2) 



e and e' being the extinction-coefficients of the two solutions. 

 From (1) we find 



cl dV ,o\ 



"bgT" logI /S U 



and from (2) and (3) , finally, we have 



That is, the concentration is proportional to the extinction- 

 coefficient, and - = A. A is, if Beer's law is true, a constant 



only dependent on the wave-length of the light and on the 

 nature of the substance. The quantity A is called by 

 Yierordt the absorption-ratio (Absorptions-verhaltniss). 



The following example will make the method of reducing 

 the results clear : — 



Solution of CuCl 2 , concentration = 2"113 gr. eq. in 1 litre. 



Trough 7*64 centim. long. 



The apparatus-correction a was found 1*003, as a mean of 

 25 readings in various positions in the spectrum. 



The following readings were taken with the solution in the 

 trough, in the region between X 518*2 and X 502*3. 



Breadth of 

 upper slit. 



Lower slit 

 Upper slit 



Number of 

 Readings. 



Greatest 



difference 



between two 



Readings. 



Mean (b). 



140 

 100 



•481 



•487 



5 

 5 



8-8 "I 



2-3 J 



0-485 



The refractive index of the ends of the trough was 1*526. 

 ,, „ of the solution was . . 1*3(>2, 



