THE ACTION OF CHEMICAL AFFINITY. 
201 
influenced both by the nature and by the quantity of other substances present in the 
same solution. 
Ferric Bromide. 
Experiments were also made on the ferric bromide. The iron salts were employed 
without dilution, as the bromide itself is but little redder than the chloride. Three 
equivalents of hydrobromic acid added to one of ferric nitrate produced a distinct 
red ; added to the ferric citrate they produced little change in the colour. Yet the 
bromine has evidently a great tendency to combine with the iron ; for, though the 
addition of a larger quantity of hydrobromic acid to the nitrate did perceptibly 
increase the colour, a maximum effect seemed attained when only about twelve equi- 
valents were added. The addition of twelve equivalents in the case of the citrate 
produced likewise a red tint similar to that from the nitrate. Bromide of potassium 
did not redden the ferric citrate. Numerical results could not be obtained on 
account of the paleness of the colour. 
If sesqiiioxide of iron be dissolved in hydrobromic acid, a very deep red solution is 
obtained, which is scarcely affected in colour by the addition of any potash salts in 
any quantity, unless, indeed, they decompose it with the formation of a red precipi- 
tate. Strong citric acid even has little effect upon it. It gives an intense blue with 
ferrocyanide of potassium in oxalic acid. This compound, however, is not a true 
ferric salt; it is an oxybromide. 
The Ferric Salts in general. 
Effect of mass of solvent . — In connexion with these experiments on ferric salts, it 
became a matter of interest to ascertain whether changes in the mass of water itself 
had any influence on the composition of the salts contained in these coloured solu- 
tions. 
The only methods which occurred to me of obtaining an answer to this inquiry, 
were, to ascertain whether dilution caused any greater or less decrease of colour in 
some substances than in others of the same tint ; and whether the decrease of colour 
by dilution was uniform in the same salt, by whatever mixture it might be produced. 
It has frequently been noticed that a red solution of ferric sulphocyanide is reduced 
by the addition of water more than the simple dilution seemed capable of account- 
ing for, and more than the red meconate is. In examining this matter, it seemed 
desirable first to ascertain how far it might be the fact. For this purpose two solu- 
tions were taken ; the one of the purest sesquisulphocyanide of iron I could obtain, 
the other of pure ferric meconate. They were made up to the same colour and the 
same volume, and were then equally diluted. It became at once evident that the 
addition of water produced far greater difference in the colour of the sulphocyanide 
than in that of the meconate ; but the exact proportion could not be determined, 
as, although the two reds were almost identical in shade at first, the sulphocyanide 
