1911-12.] Absorption of Light by Inorganic Salts. No. V. 47 
Cupric salts have been investigated with a spectrophotometer in the 
visible spectrum by Ewan,* Griinbaum,]- and E. Muller, j The results in 
Ewan’s last paper go furthest into the red, and I have plotted them with 
my results for the sake of completeness. Ewan’s values are represented 
by o’s. His solutions were about ten times as dilute as mine. 
The work done by Ewan and Muller in the visible spectrum has shown 
that for dilute solutions A has the same values for all four salts, and that 
for strong solutions its values do not differ much from one another and 
from the values for the dilute solutions, except in the violet in the case 
of the chloride and bromide, where a marked absorption grows up with 
concentration and temperature. Unlike the similar change in cobalt and 
nickel, it is here more marked in the case of the bromide. 
After finishing the four copper salts I did the sulphates of lithium, sodium, 
potassium, rubidium and caesium, and then sodium chloride, sodium bromide, 
sodium nitrate, and silver nitrate. Aqueous solutions of these salts are of 
course colourless in the visible spectrum, and I found that none of them 
absorbed at all in the infra-red. In fact they absorbed less than water, and 
hence gave rise to an apparent negative absorption. The following table 
gives the strength of the solution used in each case and the value below 
which the molecular extinction coefficient must lie in the infra-red : — 
c. 
A. 
Lithium sulphate ..... 
1-88 
•009 
Sodium sulphate ..... 
•406 
•02 
Potassium sulphate . 
•425 
•03 
Bubidium sulphate .... 
1*17 
•004 
! Caesium sulphate .... 
1-17 
•004 
Sodium chloride ..... 
3-44 
•002 
Sodium bromide ..... 
4-98 
•003 
Sodium nitrate ..... 
5-69 
•002 
1 Silver nitrate ..... 
1 
2-98 
•01 
The results in the ultra-violet in the case of the two nitrates are shown as 
curves, and in the other cases are given in a table. In the case of AgN0 3 
c was *059, and in the case of NaNO s c was T18. 
* Phil. Mag. (5), xxxiii. p. 317 (1892); Proc. Roy. Soc., lvi. p. 286 (1894); ibid., lvii. 
p. 117 (1894). 
t Inaug. Diss., Berlin, 1902. 
I Ann. d. Phys. (4), xii. p. 767 (1903); ibid. (4), xxi. p. 515 (1906). 
