30 ABSORPTION SPECTRA OF SOLUTIONS. 



narrows somewhat from strip to strip in this direction. The region of 

 transparency beyond this N0 3 band, as we shall designate it, shows that 

 the ultra-violet absorption of cobalt nitrate decreases with increasing 

 dilution, when the conditions for Beer's law obtain. If it were not for 

 the N0 3 band we should expect the ultra-violet end of transmission to 

 look very much as it did in the case of cobalt bromide, except that in the 

 present case it would be shifted somewhat towards the longer wave-lengths. 



The absorption band in the green presents a different appearance from 

 what it has done in the solutions already studied. In both the chloride 

 and bromide solutions the band narrowed rapidly at first, and almost 

 symmetrically. Here it narrows uniformly from strip to strip in A, and 

 much more from the violet than from the red side. The result is that if 

 measurements are made on the edges and calculations made for the posi- 

 tion of the center of the band, this shifts continually towards the red with 

 increasing dilution. The extreme limits of transmission in the most con- 

 centrated solution, as shown by the strip nearest the scale, are A 4450 and 

 A 5500, locating the center at A 4975. For the most dilute solution of set A 

 the corresponding numbers are ^ 4700 and ^ 5450, center at /} 5075. In set 

 B the band remains of sensibly constant width, its center falling at A 5150. 



The concentrations used in making this spectrogram were almost exactly 

 the same as those employed in making the negatives for Plate 10, hence 

 the two spectrograms are directly comparable. The comparison shows 

 that for solutions of 1.5 to 2.0 normal concentration, the width of the band 

 is approximately the same for the two salts; the band in the nitrate solu- 

 tion was, however, located nearer the region of short wave-lengths. As 

 the concentration decreases the absorption of the bromide solution becomes 

 much less than that of the nitrate solution, the band in the latter being 

 still slightly more refrangible. 



The solutions transmitted the red as far as A 7400 without sensible 

 absorption. Since the most concentrated solutions looked purple in the 

 bottles, it is probable that they exerted some general absorption in the 

 orange and red, but examination of the light transmitted through the 

 bottles, where the layer was 5 cm. in depth, with a direct-vision spectro- 

 scope failed to reveal any definite bands in this region. 



COBALT NITRATE IN WATER MOLECULES CONSTANT. (See Plate 20.) 



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 2.05, 1.67, 1.25, 0.94, 0.71, 0.55, and 0.44, the corresponding 

 depths of absorbing layer being 3, 4, 6, 9, 13, 18, and 24 mm. The con- 

 centrations for set B were 0.55, 0.44, 0.32, 0.23, 0.17, 0.13, 0.105; the 

 depths of cell were the same as for A. The width of slit and exposures 

 were the same as in the case of Plate 19. 



In A the ultra-violet transmission is limited by the red edge of the 

 N0 3 band. In the present case this band widens somewhat with decreas- 

 ing concentration, the limits of transmission being A 3280 and X 3330 for 

 the most concentrated and most dilute solutions, respectively. In B there 

 is some transmission beyond the NO 3 band, the violet limit of the trans- 



