AS MEASURED BY MEANS OF THE RADIOMICROMETER. 65 



spectrum, where such wide slits had to be used on account of the small 

 amounts of energy in this region, these ratios are not comparable. 



For this reason we have confined the larger part of our work on neo- 

 dymium compounds almost entirely to wave-lengths greater than0.7^. In 

 all the following tables and curves representing these data, constant slit- 

 widths of 0.2 mm. have been used. This was the minimum width which 

 could be employed, in order to get reasonable deflections throughout the 

 spectrum from 0.7// to 1/x. Experiments have shown that any error result- 

 ing from slit-widths would not amount to more than 3 or 4 per cent through- 

 out this region. 



The source of light was, as in the previous chapters, a Nernst glower carry- 

 ing about 1.2 amperes, and the current kept constant by means of an adjust- 

 able slide-wire resistance. The source of current was a large number of 

 storage cells, and this was never allowed to vary over 0.01 ampere. Great 

 care was exerted in keeping the current constant while obtaining a single 

 ratio, since this is really the only time in which a slight change in current 

 density was dangerous. 



DISCUSSION OF TABLES AND CURVES. 

 NEODYMIUM CHLORIDE IN WATER. 



Table 12 gives the observed transmissions of solutions of neodymium 

 chloride in water. In all the tables the following four dilutions have been 

 studied, the depths of cell being, generally, 2.5, 5, 10, and 20 mm., respec- 

 tively; and the concentrations being made so as to keep nXd = k. In 

 column 1 of each table there is given X, taken at such intervals as the solu- 

 tion in question required. In those portions of the spectrum where the 

 transmission was complete, or very nearly so, these intervals were greater 

 than in those regions where there were absorption bands. 



Reading from left to right in this table, beginning with column 2, there are 

 given the absorptions for solutions of the following concentrations: 2.141, 

 1.07, 0.535, and 0.267 normal, respectively; the corresponding depths of 

 absorbing layer being 2.5, 5, 10, and 20 mm., respectively. In every case 



x-\-\ 

 the transmission was obtained from the ratio - , where x is 2.5, 5, 10, 



and 20 mm., respectively. 



In all cases the concentrated mother-solution was carefully made up, its 

 concentration determined by a gravimetric precipitation of the metal, and the 

 succeeding solutions made by diluting measured parts of the mother-solution. 



Observations are given here over only that portion of the infra-red spec- 

 trum from X6800 to X10000. It is in this region that the most pronounced 

 neodymium bands occur. It was thought advisable not to go further into 

 the infra-red, since bej'ond lfx the general absorption due to the solvent is 

 very marked. This would, of course, interfere with a quantitative study of 

 any band occurring in this region, since it is impossible to separate the two 

 absorptions, previous work having shown that they are not additive. 



