Mr. E. Solly on the Colour of Solutions of Chloride of Copper. 367 

 following very simple equation : 



7rlogsec# = / dxy tan x; . . . (32.) 



in which y is connected with x and h by the relation 



sin y (h sin x\ 2 . 



• , y r=( — J. . . . 33. 



sin (2 x — 3/) \ x / v ' 



Observatory of Trinity College, Dublin, 

 October 6, 1843. 



XL VI. Note o? i the Changes in Colour exhibited by solutions 

 of Chloride of Copper. By E. Solly, F.R.S. In a Letter 

 to Richard Taylor, Esq. 



THE colour of the different salts of copper varies, it is well 

 known, according to the proportion of water which they 

 contain; thus the sulphate when anhydrous is perfectly white; 

 whilst in its ordinary crystallized state, when containing five 

 atoms of water, it is of a deep blue colour; most of the other 

 soluble salts of this metal exhibit similar changes in colour ac- 

 cording to their state of hydration, none perhaps exhibit them 

 in a more remarkable manner than the chloride. I was re- 

 cently misled, through ignoranceof one of the conditions which 

 modify these changes ; and as the colour of solutions of copper 

 has sometimes been supposed to indicate the state in which the 

 metal existed in solutions, I have thought that a brief state- 

 ment of the appearance of chloride of copper under different 

 circumstances might save others from falling into the same 

 error. Dry chloride of copper is of a yellowish-brown colour ; 

 when dissolved in a small quantity of water it forms a dark 

 brown solution ; if further diluted it becomes grass-green ; and 

 if more water yet be added, the solution becomes bright blue. 

 If, on the other hand, a blue and dilute solution be gradually 

 evaporated, it passes through the various shades of green-blue, 

 blue-green, green and brown. When in place of slowly eva- 

 porating a blue and dilute solution it is suddenly heated, it 

 immediately begins to change colour, and when boiling is 

 bright green. This effect appears to be due to heat alone; 

 that it is quite distinct from the change from blue to green, 

 caused by concentrating the solution, is proved by the fact, 

 that if a blue solution be made to boil in a flask it becomes 

 perfectly green whilst boiling, but regains its original blue 

 colour if allowed to cool ; this change from blue to green, and 

 vice versa, may be repeated any number of times at pleasure, 

 by alternately heating and cooling the solution, and without 

 concentrating or diluting it. 



