Manchester Memoirs, Vol. liii. (1909), No. 18. 3 



sulphocyanide, but, as their colours are not nearly so 

 intense, the effect of dilution is not so striking. Ferric 

 salicylate, which has a deep purple colour, behaves on 

 diluting with water almost in the same way as the sulpho- 

 cyanide, although the latter appears to dissociate some- 

 what more readily. 



The colour of a solution of ferric salic}'late is not very 

 unlike that of a solution of potassium permanganate, and 

 this enables us to see the remarkable difference in the 

 effect of diluting the two solutions. (Diluting a solution 

 of a permanganate will, of course, increase its dissociation ; 

 but here the negative ion is coloured. In the other cases 

 there can be no colour unless some of the undissociated 

 compound is present.) If we take a solution of potassium 



( N\ 



permanganate I say about I, two or three drops of it 



will impart a perceptible purple colouration to half a litre 

 of water. I f, on the other hand, we prepare a solution of 

 ferric salicylate, which has about the same colour intensity 

 as the permanganate, seven or eight cubic centimetres of 

 it are required to produce a perceptible purple colouration 

 in the same amount of water. It is not so easy to compare 

 the colour intensity of ferric salicylate with that of the 

 permanganate, but it is possible to prepare .solutions of 

 the two which have approximately the same colour inten- 

 sity, and in this case the difference in the amounts of the 

 two solutions which are required to produce a perceptible 

 colouration in the same quantity ot water, is even greater 

 than in the case of the permanganate and the ferric 

 salicylate. 



