CHAPTER II. 



EXPERIMENTAL METHODS AND APPARATUS. 



In this work the mapping of the spectra was done in the same way and 

 by the same methods as already described in Publication No. 130 of the 

 Carnegie Institution of Washington, pages 19-22. The general arrangement 

 of apparatus is given in fig. 4. 



New difficulties that required special treatment quite frequently presented 

 themselves in the work. As an example, one might take that of mapping the 

 absorption spectrum of solutions of samarium, dysprosium, and gadolinium 

 salts, very kindly lent us by Professor Urbain. In order to bring out as 

 many bands as possible, the absorption cell was made wedge-shaped, the apex 

 of the wedge being about 2 mm. wide and about 15 mm. long. In this way a 

 much greater cell depth could be obtained with the same amount of solution 

 than with the Uhler cell. Another difficulty consisted in getting the strip of 

 the photographic film uniformly exposed. The appearances of bubbles, of 

 precipitates, etc., during the heating of the solutions are further examples. 

 In some cases these difficulties could be overcome. For instance, if the Uhler 

 cell was moved back and forth in the path of the beam of light, bubbles and 

 precipitates only decreased the amount of light passing through the solution 

 and did not cause an uneven exposure on the strip. 



Part of the work consisted in extending the Beer's law tests to very dilute 

 solutions. Some work was done on uranyl solutions, using a trough with 

 plane parallel ends. Other cells consisted of glass tubes with quartz lenses at 

 the ends. One of these was 500 cm. long. With cells of this kind it is impos- 

 sible to obtain a uniform exposure, unless the cell is moved back and forth in 

 the path of the beam of light. 



Salt solutions that have bands in the violet and ultra-violet were exposed 

 for much longer intervals of time than those which have bands only in the 

 visible part of the spectrum. In making an exposure of this kind (the uranyl 

 salts are typical examples) the length of exposure to the shorter wave-lengths 

 would be from 5 to 10 times longer than to the longer wave-lengths. First, 

 an exposure would be made to the whole spectrum of the Nernst glower ; then a 

 screen would be placed in front of the plate, cutting out all light of wave-length 

 greater than about \ 4500 or X 4800. A long exposure would then be made to 

 the short wave-length spectrum of the glower; and lastly, a short exposure 

 would be made directly to the spark. 



For high-temperature work on acid solutions the fused silica cell was used, 

 while for room temperatures this cell and the Uhler cell were used for such 

 solutions. 



Part of this investigation consisted in extending the work on absorption 

 spectra to high temperatures, by means of closed cells. Two cells, one 1.0 cm. 

 and the other 10 cm. in length, were used. Fig. 3 represents a longitudinal 

 section of the longer cell. Since both cells were exactly alike in all respects 



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