37 
1912-13.] The Absorption of Light by Inorganic Salts. 
lumps, somewhat hygroscopic, and becoming deep golden in colour on 
absorbing moisture from the air. Ferric bromide was a dark red powder 
covered with a greenish sheen where exposed to the air ; it was exceedingly 
deliquescent. When making solutions, the salt was placed in a weighed 
beaker and a current of hot air was blown on to it in order to drive off 
moisture before it was weighed. The salt on dissolving in water gave out 
a great amount of heat, and the solution being filtered, a considerable 
brown residue was obtained. Ferric nitrate consisted of pale purplish, almost 
colourless crystals, which became somewhat moist after standing in the 
bottle for some time. The salt had a distinct odour of nitric acid. Ferric 
sulphate was a dry white powder which dissolved very slowly in water. 
The colours of the aqueous solutions of the chloride, nitrate, and sulphate 
varied with diminishing concentration from reddish brown to light yellow, 
but the more concentrated solutions of the bromide were very dark brown, 
almost black, in colour. 
Ferrous chloride consisted of pale green hexagonal plates which became 
darker in colour, brown in parts, on standing for some time. Ferrous 
sulphate consisted of clear blue-green crystals, not very soluble in water. 
The aqueous solutions of the ferrous salts were green, but not deep in colour. 
They decomposed, owing to oxidation, soon after being made up, a flocculent 
brown precipitate of Fe 2 0 3 forming in them. 
The formulae of the different salts examined were taken as : 
Fe 2 Cl 6 . 12H 2 0 Fe 2 (NOo) fi . 18ELO FeCL 
Fe 2 Br 6 " Fe 2 (S0 4 ) 3 “ FeS0 4 .7H 2 0. 
A solution of concentration c = 1 was taken as one in which the number of 
grammes of the salt dissolved in 1 litre of solution was that represented by 
the corresponding formula. 
The apparatus used for making measurements in the visible spectrum 
was the same as has been described in the second of this series of papers.* 
Measurements in the infra-red were made with the mirror spectrograph 
described and illustrated in the fifth article of the series.]* Preliminary 
measurements of absorption in the ultra-violet were made in the case of the 
ferric salts with the aid of the apparatus used in the case of the cobalt and 
nickel salts.J It was found, however, that the absorption increases so 
rapidly in the ultra-violet that it could not be measured at all accurately 
except for very dilute solutions and for very small thicknesses of solution. 
Quantitative measurements in the ultra-violet were therefore not carried out. 
* Proc. Roy. Soc. Edin ., xxxi. p. 530, 1911. + Ibid., xxxii. p. 41, 1912. 
f Ibid., xxxi. p. 547, 1911. 
