39 
1912-13.] The Absorption of Light by Inorganic Salts. 
the apparatus at my disposal. The solution of the sulphate of concentra- 
tion c = l - 0 was nearly at the limit of solubility, but solutions of concen- 
trations c = 4*0 and c = l*0 were experimented with in the case of the 
chloride. Various thicknesses of solution were used, varying from 
d — ' 14 cm. to d = 2-0 cm. 
In order to facilitate comparison, the chief results are shown in graphical 
form in figs. 1, 2, and 3. It may be observed that with the ferric salts the 
bands at the violet end of the visible spectrum commence at about *560 jul. 
Hiebendaal * examined the absorption spectrum of ferric chloride in alcohol, 
and found bands at *680, ’620, *595//,, and complete absorption from about 
'555/x. The first three bands do not occur in the aqueous solutions. We 
see also from the curves that in the case of ferric chloride, nitrate, and 
sulphate the infra-red absorption band increases in height and the maximum 
point of the band shifts somewhat towards the visible spectrum as we pass 
from the concentration c = 1*0 to the concentration c— T, whereas with 
ferric bromide the shift of the maximum of the band is in the same 
direction, but the height of the band decreases with dilution. The bromide 
bands are much higher than the corresponding bands of the other three 
ferric salts ; this may probably be due to the presence of free bromine in 
solution. Further, it may be observed that the ferrous chloride infra-red 
band does not alter much with change in concentration. 
The effect of concentration on the absorption was next more fully 
investigated. In the case of the ferric salts, solutions of concentrations 
c = 10, '7, ’4, *1, and ’01 were examined for three wave-lengths in the 
visible spectrum, namely, *453, *529, and *653^. Solutions of ferrous 
chloride of concentrations c — 4H, 2 - 0, 1*0, and ’5, and of ferrous sulphate of 
concentrations c = l’5, *7 5, and *5, were also examined for the same three 
wave-lengths. The measurements were made on these solutions when they 
had been freshly made up, and were repeated in the case of the ferric salts 
after a period of six weeks in order to see if any change in the absorption 
could be detected. 
The results of the measurements are given in Table II. below. The 
numbers in the first column in each case are the values of the molecular 
extinction coefficient A for freshly made-up solutions, while the numbers 
in the second column are the values for A for the same solutions after 
six weeks. 
Most of the above results are shown in graphical form in figs. 4, 5, and 
6. The dotted curves represent the measurements on the six-weeks-old 
solutions, while the other curves represent the measurements on the fresh 
* Hiebendaal, “ Onderzoek over eenige absorbiespeotra,” Academ. Proefschrift , 1873. 
