224 On the Colour Relations of Copper and its Salts. 



intense in the light reflected from copper than in the light re- 

 flected from paper. The other parts of the spectra were of 

 equal intensity in each. The red in the spectrum from copper 

 is lengthened out beyond the point at which the red disappears 

 in the spectrum obtained from white paper or direct from a 

 window. As far as could be determined with the instrument 

 at my disposal, the sodium-line exactly forms the boundary of 

 the absorption-band of copper in solution, and of the bright- 

 red region in the spectrum of light reflected from the metal. 



The latter spectrum possesses all the characteristics of an 

 ordinary one, with this exception, that its red region is inten- 

 sified and somewhat lengthened out. 



These results confirm the conclusions drawn from the expe- 

 riments described as made with the tube. In those experi- 

 ments the excess of red light was absorbed by the metal in 

 solution and white light passed through. Several preliminary 

 attempts to found a method of estimating copper upon these 

 properties were made as follows : — Three tubes, similar to the 

 first described, were placed parallel and vertical above a po- 

 lished sheet of copper ; they were protected from extraneous 

 light by a cylinder of blackened card, closed at the bottom by 

 a piece of card also blackened, and pierced by three holes for 

 the passage of the tubes. The tubes were graduated from the 

 bottom upwards. In one tube a column of the dilute copper 

 solution was placed of sufficient length to just allow the colour 

 of copper to pass through, while the column in another tube 

 was sufficiently long to cause a faint predominance of blue. 

 A portion of the solution of unknown strength being placed 

 in the third tube, it was easy to adjust its length until the 

 light passing through it was intermediate between the red 

 shade of the first tube and the blue of the second. 



Other matters having intervened, I have been unable to 

 proceed far in this direction ; but the results already obtained 

 justify the expectation that the method would be of much 

 value in approximately determining the strength of very dilute 

 solutions of copper, such as those running from mines, from 

 which the copper is precipitated by metallic iron. 



These experiments were conducted in the chemical labora- 

 tory of the Royal College of Science, Dublin, by the kind per- 

 mission of Professor Galloway. 



