1S2 HYDRATES IN AQUEOUS SOLUTION. 



a relatively small number of spectrograms were made with this cell in the 

 present investigation, it seems desirable to discuss this matter only briefly 

 in the present connection. 



Since the length of the liquid wedge at right angles to its reflecting edge 

 was about 3.5 cm. and as the length of the slit of the spectrograph was a little 

 over 1 cm., the chief operation in the manipulation of the wedge-cell consisted 

 in translating it over the slit-jaws, and parallel to the slit by proper amounts 

 between successive exposures. Experience showed that the correct distance 

 to move the cell was exactly 10.5 mm. The film-holder had, of course, to be 

 moved through corresponding distances between the respective exposures 

 just mentioned. Consequently, the resulting spectrograms consist of three 

 photographic impressions placed side by side and close together. 



The edge of a spectrogram nearest to the comparison spectrum was pro- 

 duced by light that had passed through the thinnest layer of absorbing 

 liquid (usually of zero depth), while the most remote edge resulted from 

 the radiations transmitted by the deepest layer of solution. The slit width 

 of the spectrograph was generally 0.08 mm. 



PLATES. 



Before entering upon the presentation and discussion of the experimental 

 results, a few words must be devoted to certain details that are common 

 to most or all of the plates showing the absorption spectra of the solutions 

 studied. 



In the first place, when more than eight strips belong to a plate, the cor- 

 responding photographic positives had to be made by placing the two half- 

 negatives in the printing frame, with their edges as close together as possible, 

 and with the line of the same wave-length exactly opposite to one another, 

 i. e., in proper register. In general, the line of contact of the two half-films 

 shows in the figures as a streak, of intensity different from that of the adja- 

 cent portions of the print, and extending from one end of the figure to the 

 other. It is more than likely that slight errors in wave-length have been 

 introduced in some cases by this process of reproduction, which, however, 

 was necessitated by the limited capacity of the film-holder. In most cases 

 these errors can be fully allowed for by observing the relative displace- 

 ment of the two segments of some chosen sharp emission line, which appears 

 on the two photographic strips lying on both sides of, and closest to the 

 above-mentioned streak. Whenever this line of demarkation does not 

 show distinctly on the figure, a definite statement will be made in the text 

 as to where the change from one half-film to the other is. However, in 

 cases where no emission lines are present on the two strips bordering on 

 the streak, the reader has no very good means of telling whether the half- 

 films were in exact register or not, and, therefore, he will have to assume 



