SALTS OF COBALT. 31 



mitted region remaining unchanged with dilution. This is exactly the 

 same as was found with cobalt bromide when molecules were kept con- 

 stant. This ultra-violet absorption does not vary with dilution, indicat- 

 ing that it might be a property of the molecules of the dissolved salt. The 

 NO 3 band, however, widens quite perceptibly with dilution, its limits 

 (transmission) in the most concentrated solution of B being X 2800 and 

 X 3150, while the corresponding figures for the most dilute solution are 

 /I 2700 and /I 3230. 



The green band in A widens, though not uniformly, with decrease in 

 concentration. The edges for the most concentrated solution are at A 4550 

 and A 5500, and for the most dilute solution A 4450 and X 5600, respectively. 

 It appears, therefore, that for this range of concentrations the widening 

 is symmetrical, the center of the band remaining approximately at X 5000. 

 In B the band also widens, but somewhat unsymmetrically, the violet 

 edge being displaced much more than the red edge. The center of the 

 band for the most dilute solution is at X 5175. The red is freely transmitted 

 to beyond X 7400. 



COBALT SULPHATE IN WATER BEER'S LAW. (See Plate 21.) 



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 0.65, 0.52, 0.41, 0.33, 0.26, 0.20, and 0.16; the corresponding 

 depths of absorbing layer were 6, 7.5, 9.5, 12, 15, 19, and 24 mm. For 

 set B the concentrations were 0.26, 0.20, 0.16, 0.13, 0.10, 0.08, and 0.06; 

 the depths of cell were the same as for set A. The slit was adjusted to a 

 width of 0.01 cm. and the exposures to the Nernst lamp and spark lasted 

 for 1 and 2 minutes, respectively. 



The solutions have only one absorption band, namely the one in the 

 green. In the ultra-violet they are perfectly transparent, the last lines 

 in the comparison spectrum showing with the same intensity through 

 the cell containing the solution. In the red they transmit freely light of 

 all wave-lengths as far up as X 7400. 



The band in the green has exactly the same width and position in all 

 the strips belonging to the solutions of one set; hence, Beer's law holds 

 rigidly. In set A the extreme limits of transmission are X 4900 and X 5400. 

 The shading of the violet edge is great, extending somewhat below X 4500. 

 The red edge is somewhat sharper, although not as much so as would ap- 

 pear from the spectrogram. The increasing sensibility of the Seed film 

 with wave-length in the region of X 5400 makes this edge appear much 

 sharper than it really is. 



A comparison of the seventh strip (counted from the scale) of A with 

 the seventh strip of B, Plate 3, shows that for a concentration of 0.16 nor- 

 mal, the absorption of the sulphate is slightly greater than for the chloride; 

 as the chloride deviates from Beer's law, while the sulphate solutions do 

 not. We may say that, for a certain range of concentrations greater than 

 0.2 normal, the absorbing power of solutions of the chloride and sulphate 

 is the same in the green; for greater concentrations the chloride exerts 

 stronger absorption, while for more dilute solutions the absorption of the 



