2 Taylor, Colour Demonstrations of Water. 



both dissociate, and both yield the same copper ion, which 

 is blue. 



The above examples are all well known, but they are 

 all connected with salts which dissociate when in solution 

 yielding coloured ions. But we know a number of com- 

 pounds which are themselves highly coloured, and are 

 soluble in water, but which can be made from colourless, 

 or almost colourless, materials. For example, ferric sul- 

 phocyanide is a compound with a deep blood-red colour, 

 which is produced when a solution of a sulphocyanide 

 (colourless) is added to a solution of a ferric salt (which 

 when dilute has a pale yellow colour). 1 he compound is 

 very soluble in water. According to the theory, what 

 ought to happen if this highly coloured compound is 

 diluted with much water? If it dissociates, its ions are 

 all but colourless, so that the blood-red colour ought to 

 disappear. This is exactly what does occur. If some of 

 the deep red liquid is poured, a little at a time, into a 

 considerable bulk of water (one or two litres), it is very 

 striking to notice the complete disappearance of the colour 

 of the first portions added, and the really considerable 

 amount which has to be added to produce a permanent 

 red colouration in the water. 



If an amount of the coloured liquid, just insufficient 

 to produce a permanent red colouration, has been added 

 to a considerable volume of water, it is interesting to note 

 that the red colour appears on the addition of either solu- 

 tion of potassium sulphocyanide, or ferric chloride. The 

 existence of the red compound depends on the concentra- 

 tion of the ions Fe and (CNS'), and if the concentration 

 of either of these ions in the solution is increased, some 

 of the red compound is formed. 



The red compounds ferric acetate and ferric formate, 

 when diluted with water, behave in a similar way to the 



