56 ABSORPTION SPECTRA OF SOLUTIONS. 



the NO 3 band, or very nearly so for solutions of the concentration here 

 used, since there is little or no narrowing of the absorption in passing from 

 the fifth to the seventh strips of A. In B the transmission is sharply 

 limited by the NO 3 band throughout, transmission ceasing at A 3280. 



It is interesting to compare the absorption of this salt with that of 

 copper chloride. The latter in concentrated solutions absorbs not only 

 the ultra-violet, but also all of the violet and blue. It must be evident, 

 then, that all the absorption in the blue, violet, and perhaps also the ultra- 

 violet, in solutions of the chloride and bromide, is to be ascribed to the 

 molecule, or to the molecule and whatever may be associated with it, and 

 not in any way to the ions. The copper ions very likely exert no absorp- 

 tion on light of such short wave-lengths as come within the range of the 

 present investigation. 



In the red the most concentrated solution of A absorbs everything of 

 wave-length longer than X 5980, the most dilute solution of A transmitting 

 some light as far out as X 6250. For B the limits of transmission for the 

 most concentrated and most dilute solutions are, respectively, X 7200 and 

 X 7250, there being considerable shading from about X 6600 in both cases. 



Again comparing the absorption of the nitrate and the chloride, making 

 due allowances for differences in concentration, we find that the red band 

 is sensibly the same for the two salts emphasizing again the fact that 

 the absorption of red is chiefly a function of the concentration of copper 

 atoms, depending only to a slight extent on their immediate surroundings. 



COPPER NITRATE IN WATER MOLECULES CONSTANT. (See Plate 45.) 



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 4.04, 3.18, 2.36, 1.78, 1.38, 1.11, and 0.92; the corresponding depths 

 of absorbing layer were 3, 4, 6, 9, 13, 18, and 24 mm., respectively. The 

 concentrations for B were 1.00, 0.81, 0.60, 0.46, 0.347, 0.273, and 0.223; 

 the depths of absorbing layer were the same as in A. The exposures to 

 the light of the Nernst lamp and spark lasted for 1 and 3 minutes, respec- 

 tively, the slit being adjusted to a width of 0.01 cm. 



A shows that the absorption in the ultra-violet still narrows with dilu- 

 tion, the limits of transmission for the most concentrated and most dilute 

 solutions being, respectively, X 3600 and X 3430. In B the transmission is 

 limited by the NO 3 band, the edge of which falls at X 3280 throughout. 

 Since the ultra-violet absorption narrows even in this, it is evident that 

 something in addition to the simple theory of dissociation is needed to 

 account for the facts. 



In the red we find that the band first narrows until the third strip of 

 A is reached, then widens continuously with dilution, the edge forming a 

 curved line concave towards the violet. The limit of transmission for the 

 first solution is X 6000, for the third X 6050, and for the seventh it is at 

 X 5950. In B the absorption increases regularly with dilution, the limit 

 in the first solution being at X 6800 and at X 6650 for the seventh. There 

 is considerable shading, but this also increases somewhat with decrease 

 in concentration. 



