SALTS OF COPPER. 51 



The exposures to the light of the Nernst lamp and spark lasted 1^ and 

 3 minutes, respectively, the slight increase in the length of exposure to the 

 Nernst lamp being occasioned by the fact that the line voltage had dropped 

 so low that it was no longer possible to keep the current through the lamp 

 at the usual value of 0.8 ampere. It was accordingly kept at 0.76 am- 

 pere, and the time of exposure lengthened as indicated. Before any more 

 work was done, the line voltage was permanently raised by an adjustment 

 of the step-down transformers, so that the current could always be kept 

 at 0.8 ampere. 



The solution in pure acetone absorbed completely all wave-lengths 

 shorter than A 5250. As the percentage of water was increased the absorp- 

 tion moved rapidly towards the violet at first, then more slowly, becoming 

 almost stationary towards the last. The second, third, and fourth strips 

 show the presence of the absorption band at A 4730. This, however, dis- 

 appears very rapidly with addition of water. The limit of transmission 

 for the fifth solution, containing 4 per cent of water, is at A 4500, while for 

 the sixteenth, with 30 per cent of water, it is at A 4350. 



In the red the absorption is slight. The solution in pure acetone, how- 

 ever, shows considerable general absorption throughout the entire red 

 region, and also indicates the presence of a band beyond A 7000. This 

 band narrows with increase in the percentage of water, no sign of it being 

 visible in the strip corresponding to the seventh solution. 



COPPER BROMIDE IN WATER BEER'S LAW. (See Plate 38.) 



The concentrations of the solutions used in making the negative for 

 A, beginning with the one whose spectrum is adjacent to the numbered 

 scale, were 1.08, 0.72, 0.50, 0.36, and 0.27, the corresponding depths of 

 absorbing layer being 6, 9, 13, 18, and 24 mm. For B the concentrations 

 were 0.18, 0.12, 0.083, 0.06, and 0.045, the depths of cell being the same 

 as for A. The concentration of the mother-solution of copper bromide was 

 2.16, but its color was so deep-brown, and there was such an amount of 

 general absorption throughout the spectrum, that it was impossible to get 

 any light of sufficient intensity to affect a photographic plate through a 

 layer of it having a depth of 3 mm. Even when diluted to 1.62 normal 

 a layer of 4 mm. in thickness absorbed practically everything except a 

 limited region in the orange-red. The solution whose concentration was 

 1.08 had a dark olive-green color, the change in color from 1.6 to 1.08 

 normal being very rapid and striking. With decrease in concentration 

 below 1.08 the color changed gradually to a light-blue. 



The exposures to the light of the Nernst lamp and spark lasted, respec- 

 tively, 1% and 3 minutes, the slit having the usual width of 0.01 cm. 



The bands in the ultra-violet and red both narrow considerably with 

 decrease in concentration, the narrowing being very much greater for the 

 band of shorter wave-lengths. The most concentrated solution in A ab- 

 sorbed everything of shorter wave-length than X 4550, while the most 

 dilute solution transmitted as far down as /I 3900. For B the correspond- 

 ing limits are A 3400 and X 3120. The line formed by the limits of trans- 

 mission is visibly curved, the convex side turning towards the region of 



