148 Proceedings of the Royal Society of Edinburgh. [Sess. 
The next point investigated was in connection with the so-called 
“ colour of the ions.” Various physical chemists have ascribed the colour 
Values of A for the Acetates. 
a. 
703 up. 
670 
640 
614 
592 
573 
556 
540 
525 
512 
502 
492 
482 
473 
465 
457 
450 
443 
438 
Cobalt. 
c = -288. 
•22 
•27 
•33 
•44 
•68 
1*22 
2-47 
4- 37 
5- 94 
6*30 
5*84 
5-48 
4-99 
4-29 
367 
2*79 
2*15 
1-63 
1-10 
Nickel. 
c=-264. 
2-35 
2*26 
1-80 
1-07 
•54 
•31 
•22 
17 
•14 
•13 
•09 
•12 
•21 
•32 
•42 
•51 
•62 
•88 
1-31 
Copper. 
c = *068. 
20-4 
15-5 
10-6 
6-21 
383 
2-56 
1-70 
•98 
changes of cobalt, nickel, and copper salts to ionisation. In Ostwald’s 
Solutions , translated by M. M. Pattison Muir, 1891, pp. 269-270, we find 
stated : — 
“ The colours of salt solutions are essentially the colours of the parts 
of molecules, or ions, contained therein, and all salt solutions which contain 
a certain ion must exhibit the characteristic colour of that ion. Should 
the expected colour not appear, we may conclude that the corresponding 
ion is absent. 
“ The red colour of dilute solutions of cobalt salts, for instance, indicates 
the presence of cobalt ions. The sulphate, nitrate, chloride, etc., have the 
same colour in solution ; the colour is independent of the nature of the 
negative ions. If one of these solutions is boiled with an excess of potassium 
cyanide, it is decolorised, and the colourless solution no longer shows the 
reaction of cobalt. The compound potassium cobalticyanide, K 3 Co(CN) 6 , 
has been formed ; free cobalt ions are no longer present. The green colour 
of a solution of a nickel salt is changed, under the same conditions, to 
yellow, which shows that the nickel ions have entered into combination. 
“ If the foregoing statement is correct, the intensity of the coloration of 
those salts whose ions are coloured must be proportional to the quantity 
