218 HYDRATES IN AQUEOUS SOLUTION. 



length 2980.8 was the most refrangible radiation recorded by the negative 

 for the first solution of the series. The continuous spectrum did not extend 

 beyond 0.305/1. For the 7th solution transmission began at about 0.400/. 

 The negative strip for the Sth solution recorded very faint transmission from 

 0.436, on in the direction of greater wave-lengths. A minimum was at 0.518.". 

 and strong transmission began in the neighborhood of 0.527/<. As often 

 emphasized in preceding paragraphs, great care must be taken to avoid 

 drawing false conclusions from photographic records for which unavoidable 

 under-exposure permitted the variations in sensibility of the photographic 

 emulsion to exert undue influence on the record. 



The absorption spectra shown by plate 15 (a) pertained to the 6th. 7th, 

 Sth, 9th, and 10th solutions of the complete set now under discussion. The 

 photographic strip farthest away from the comparison scales corresponds to 

 the most dilute solution of the group whose spectra are given by the plate. 

 The depth of the cell was 0.08 cm. The glower exposures were each 2.5 min- 

 utes long. The negative strip corresponding to the most dilute member of 

 this group of five solutions showed that transmission began very weakly near 

 0.380/'.. It indicated no general absorption in the orange. The third strip, 

 i. e., the one pertaining to the solution of concentration 2. 033, show r ed on the 

 negative that faint transmission began at 0.434/<, rose to a maximum at 

 0.462/t, then faded out to a minimum of almost complete absorption at0.495/<, 

 and finally became strong at about 0.53, and so continued to the end of the 

 negative. The fourth strip which pertained to the solution of concentration 

 2.542 showed very faint transmission from 0.56S/< to the end of the negative. 

 The negative recorded nothing for the solution of concentration 3.050. 



Plate 16 (&) gives the absorption of the solution which contained the largest 

 amount of calcium bromide (of concentration 3.804) when placed in the wedge- 

 shaped cell. The liquid prism showed a dark, reddish-brown color. The 

 edge of the spectrogram nearest to the comparison spectrum corresponds, 

 of course, to the least thickness of absorbing layer. The angle of the wedge 

 was about 15.6', and hence, since the cell was adjusted to begin at zero depth, 

 the greatest thickness of absorbing layer was about 0.14 mm. The negative 

 recorded the strong line of wave-length 2265.1 as transmitted by the thinnest 

 portions of the solution, but nothing more refrangible. The continuous 

 background did not extend to shorter radiation than 0.233/<. The ultra- 

 violet absorption is seen to be very intense when the small thickness of the 

 solution is taken into account. The opacity of the solution began to 

 decrease in the neighborhood of 0.346, and rose gradually to a minimum at 

 4525. The negative recorded the maximum of absorption in the green as at 

 wave-length 0.515/<. The spectrogram just referred to is supplemented by 

 plate 16 (a). The negative of which this plate is a reproduction was made 

 with a Cramer trichromatic plate. The solution used was the same as 



