BY MEANS OF THE RADIOMICROMETER. 13 



If equal depths of a concentrated aqueous solution were used in 

 place of equal depths of pure water, a number of precautions were 

 necessary in order that the same deflections would be obtained with 

 the two cells. It was, of course, necessary that the solutions in both 

 cells should remain perfectly clear. If even the slightest opalescence 

 developed in one of the cells, the transmission in this cell would be 

 less than in the other. It was further found that special precautions 

 had to be taken to keep the glass ends of the cells clean. The ordi- 

 nary methods of cleaning did not suffice. In filling the cells with a 

 solution, some of the solution will usually come in contact with the 

 outer surface of the glass plate in the larger cylinder. Even after 

 considerable rubbing with a cleansing-cloth there will remain an almost 

 invisible film of the crystallized salt on the plate. This was found to be 

 sufficient to change the transmission as much as 5 per cent. Special 

 precautions must be taken to clean the ends, otherwise comparable 

 results could not be obtained. 



EARLIER RESULTS. 



It was earlier found by Jones and Guy 1 that when the product of 

 the concentration of the solution multiplied by the depth of layer was 

 kept constant, Beer's law did not hold for neodymium salts. Certain 

 tentative suggestions were made in that work which, in the light of the 

 present results, must be somewhat modified. The point in question 

 involves the increase in the intensity of the neodymium bands with 

 dilution. A possible explanation of this phenomenon, based upon 

 resonance, was offered. 



It was later found that there should be some correction made for 

 the water absorption in the case of the 0.87^t band, since here the 

 IM water-band began to absorb slightly. If we take into account both 

 of these factors, together with the additional correction for the width 

 of slit 2 that was used, it is thought that the phenomenon referred to 

 above might be accounted for, and that Beer's law would hold for the 

 dilute solutions of neodymium salts. 



EFFECT OF SLIT-WIDTH. 



The effect of the width of the slit on the character of the transmission 

 curves for solutions of neodymium salts was not well understood. It 

 seemed desirable that some work should be done on this problem, with 

 the hope that we might arrive at a better understanding of this effect. 

 It was found to be especially difficult to obtain concordant results 

 when working with neodymium salts. This was due to the low dis- 

 persive power of the glass prism with which the Hilger spectroscope 

 was provided, to the difficulty in accurately setting the calibrated head, 

 and to the very narrow and sharp bands of neodymium salts. Rotating 



1 Amer. Chem. Journ., 44, 1913. 2 Carnegie Inst. Wash. Pub. No. 190, p. 70. 



