20 STUDY OF ABSORPTION SPECTRA 



METHOD OF PROCEDURE. 



Suppose we wished to study a five normal solution of calcium chloride. 

 From the known concentration of the solution and its specific gravity, 

 we can readily calculate the volume percentage of water which the 

 solution contains. At 20, the temperature at which all of this work 

 was done, it was found that the above-named solution contains about 

 90 per cent of water. The 1/z water-band is of such intensity that 

 20 mm. of water are ample for its study. The maximum absorption at 

 the center of the band, for such a depth of layer, is about 60 per cent. 

 We can study any changes better when the absorption is not so intense 

 as to give small deflections. Furthermore, the deflections given by the 

 radiomicrometer under these conditions would be sufficient to allow 

 accurate readings. In all of this work, unless otherwise stated, the 

 width of the slit was 0.2 mm., and the current 0.8 ampere. Under these 

 conditions the deflection for 20 mm. of water or solution would, at the 

 point of maximum absorption or the center of the band, be over 60 mm. 



The absolute transmission for 20 mm. of the solution of calcium 

 chloride would be determined according to the principle already out- 

 lined. Its transmission curve could then be compared directly with 

 that for 18 mm. of pure water. A layer of water of this depth absorbs 

 so intensely at 1.25/j, that it would be impossible to study under these 

 conditions the intense 1.25^ water-band. If 10 mm. of solution are 

 used, the 1.25/j water-band can be studied very satisfactorily, the 

 absorption at the center of the band being about 65 per cent and the 

 minimum deflection at this point being close to 70 mm. There are, 

 then, four transmission curves for each salt one for the solution and 

 one for the solvent as the result of studying the 1/z water-band, and 

 one for the solution and one for the solvent when studying the 1.25/i 

 water-band. 



THE RESULTS. 



Table 6 contains the results of two series of measurements with 

 magnesium chloride. The second series was made about four weeks 

 after the first. The transmissions are those of 20 mm. of the solution. 



In table 7 are given the results for two series of measurements with 

 potassium chloride. New solutions were made up for the second series 

 of measurements. These results show that the work can be repeated 

 with a fair degree of concordance. 



Tables 6 and 7 also show that there is a marked difference between 

 the transmission of the hydrated salt (magnesium chloride) and a layer 

 of water equal in depth to that in the solution. No such difference 

 exists in the case of potassium chloride, a typical non-hydrated salt. 



These differences come out most clearly for those wave-lengths of 

 light which are most strongly absorbed. Consider the region about 

 the Ifj, band, table 6, say at 1.2ju, where the deflections are fairly large 



