36 ABSORPTION SPECTRA OF AQUEOUS SOLUTIONS OF CERTAIN SALTS 



When we first began to investigate any given salt we made a preliminary 

 survey of its spectrum, noting the approximate positions of the absorption 

 lines and bands. We then made our observations very close together over 

 the regions in which the preliminary survey had indicated the presence of 

 lines and bands. The number of absorption lines and bands, as is well 

 known, is very great in the case of neodymium compounds, and these lines 

 and bands frequently have very sharp edges. This made the work with this 

 substance very difficult. The proper width of slit and position had to be 

 chosen or a considerable error would result. Given a slit width which was 

 approximately the same as that of an absorption line, a very slight move- 

 ment of the slit or prism would change very greatly the total amount of 

 energy falling on the thermal junction. 



Take the neodymium band X4275. which is very intense but narrow. On 

 both sides of this band there is a region of almost perfect transparency. If 

 the slit width necessary to give the desired deflection was greater than the 

 width of this band, light would pass through around the edges of the band, 

 and an error, which might be of very considerable magnitude, would result. 

 With substances which did not contain such fine lines and bands the work 

 would be much simpler. 



The entire spectrum from wave-lengths X4000 to X20000 was observed at 

 intervals of from 20 a.u. to 50 a.u., except in the regions where the pre- 

 liminary survey indicated the absence of absorption lines and bands. 



An examination of table 2 will show at X486 a transparency of 72 per 

 cent, which rapidly decreases, reaching the first minimum at X505. There 

 the transparency amounts to only 2 per cent. The transparency then 

 increases a little and quickly drops to 6 per cent at X515. The transparency 

 then increases, becoming nearly complete at X535. We have here, then, a 

 double band with greater absorption on the red side. Other minima appear 

 at X572, X730, X786, and X860. Bands X730, X786, and X860 do not appear 

 on the photographic plate, and the last two seem never to have been detected 

 before. The above wave-lengths are given as in the tables. 



The salts of neodymium were studied as far as X20000, but beyond 1/x 

 there seems to be complete transparency. The absorption of water is, as is 

 well known, very great in the region X12000 to X20000. 



DISCUSSION OF THE RESULTS. 



The results are plotted in figs. 1 to 11. The abscissae are percentage trans- 

 parencies, the ordinates are wave-lengths. These curves, since they repre- 

 sent the transparencies of the solutions in question, are called transmission 

 curves. 



Figs. 1, 2, and 3 represent the transparency of solutions of neodymium 

 chloride expressed in terms of Beer's law. If we represent the concentration 

 by N and the depth of layer by d, 



Nd= constant 



