The Absorption Coefficient of Solution for Monochromatic Radiation. 15 



eter. If the initial intensity, 7 , of the light falling on the cell was the 

 same in each case: 



7 = J X10-* r = I XW- ah ' /. a=^rlog 10 j (3) 



or 1 



where d and d' are the deflections produced by I and /', respectively, 

 and t is the difference in thickness in millimeters of the two cells. 

 This method eliminated all corrections for reflections from the glass 

 surfaces and thus gave a directly. 



For the study of each salt, solutions of the salt in the solvent were 

 prepared varying in concentration from saturation to moderate dilution. 

 The absorption curve for each solution was then drawn. This involved 

 the determination of a at intervals of 20//ju to 40fj.n throughout the 

 available region of the spectrum i. e., from GOO/^u to l,300ju/z. 



The experimental procedure was as follows: The two cells, filled 

 with the solution whose absorption was to be measured, were mounted 

 in place in their frame and were adjusted until the image of the Nernst 

 glower on slit A suffered no displacement when either cell was inter- 

 posed in the path of the light. The zero-reading of the radiomicrom- 

 eter was taken, and then the deflections were noted for each cell in 

 turn in the path of the light. This was done for each wave-length, 

 the shutter (s, fig. 1) being closed, usually after every four readings, 

 to see if the zero remained unchanged. Readings were taken for 

 wave-length intervals of 20/iju to 40juju throughout the entire available 

 spectrum, and the whole set was repeated in reverse order. Thus 

 each point on an absorption curve, i. e., each measurement of x, was 

 the mean of two, and often more, separate determinations. 



As an illustration of the method of procedure, the complete read- 

 ings for a solution of NiS0 4 in water are given in table 2. 



The data from which the curves have been plotted are arranged in 

 tables. For each solution the following data are recorded in these 

 tables: the temperature of the solution in degrees centigrade; t, the 

 difference in thickness of the two cells; c, the concentration in gram- 

 molecules of salt per liter of solution; the values of a calculated from 

 equation (3) ; and the values of A calculated from equation (2) . 



The short-wave limit of the absorption curves is at about GOO/*/* 

 because the deflections of the radiomicrometer for light of wave-length 

 shorter than 600/^^1 are too small to give accurate values of a. The 

 long-wave limit is at about l,200juju, although the limit set by the 

 transparency of glass is at about 2,000juju. The reason for this was that 

 in order to study regions beyond 1,200/z/i a color screen had to be used. 

 Wave-length 1,200/xju in the first-order is overlapped by wave-length 

 of the second-order. A thin layer of a strong solution of chro- 



