Intensity of Color of Solutions of Salts in Water. 417 



contains one or more colored ions, the colors of the salt and 

 the ions will mix, and the solution will have an intermediate 

 color. Any circumstance causing a change in the state of dis- 

 sociation of the dissolved salt will change the character as 

 well as the intensity of the solution. 



Of course, the more concentrated the solution of a salt, the 

 more intense its color ; but there is another way by which the 

 color may be rendered more intense, while the concentration 

 of the salt, i, e. the amount of salt to fixed amount of water, 

 does not change at all. This consists in heating the solution. 

 Abundant experimental proof of this statement is found in an 

 investigation by Gladstone,* " On the Effect of Heat on the 

 Colour of Salts in Solution," which was published in 1857. 

 Let us see why this change of intensity of color should take 

 place in heating a solution. 



It has been found that the electrical conductivity of solutions 

 of salts increases as the temperature rises, about 2 per cent 

 for each degree of temperature. In the light of Arrhenius's 

 electrochemical theory this means that salts in solution become 

 more and more decomposed into their ions as the temperature 

 rises ; the higher the temperature, the more concentrated the 

 solution with ions. Accordingly, as the intensity of the color 

 of a solution depends in a great measure upon the number of 

 ions contained in it, if a colored solution be heated, its color 

 deepens. 



Gladstone's paper commences with these pregnant sen- 

 tences : — " As a general rule, the solution of a salt has the 

 same power of absorbing or transmitting the rays of light at 

 all temperatures. I am not acquainted with any instance of a 

 dissolved colorless salt which assumes a color when the solu- 

 tion is either heated or cooled ; nor does the converse seem 

 ever to occur, — a salt colored at the ordinary temperature, 

 which loses that color when heat is applied. Nevertheless it 

 is not rare .to find colored salts which, when dissolved in water, 

 vary in shade or in tint according to the temperature. 



In some cases, heating the solution seems merely to intensify 

 the color. This is the case with the following red, orange, 

 yellow, and green salts : — 



Meconate of iron — red. 

 Terbromide of gold — red. 

 Red nitrate of cerium. 

 Bichromate of potash — orange. 

 Ferrocyanide of potassium — yellow. 

 Molybdous chloride — green." 



A number of instances are now given of changes in the 

 character as well as in the intensity of the color, when the 



* Phil. Mag., xiv, 423. 



