1910-11.] Absorption of Light by Inorganic Salts. No. I. 529 
As d was known, and as solutions were used for which A and c were known, 
A' could thus be calculated. 
The alternate heating and cooling was necessary. During a single 
heating evaporation takes place and there is a consequent increase of 
concentration. Bubbles form sometimes. Also, particularly in the case of 
the bromide and iodide, there is a formation of a precipitate. These 
changes take place in the one direction and have the same effect, no matter 
whether the solution is being heated or cooled. Hence, by taking the mean 
of the effects of heating and cooling, these disturbing influences cut out and 
we get the reversible change due solely to the difference of temperature of 
the solution. 
Only four wave-lengths were taken, but they are sufficient to show the 
nature of the change. The fluoride was not experimented on, as it is 
decomposed by heating. Two different strengths of the chloride were used. 
The. following table gives some results : — 
Increase in A produced by Heating. 
A. 
Chloride. 
c = '67. 
Chloride. 
c = 3‘10. 
Bromide. 
c = 2-78. 
Iodide. 
c = 2-96. 
Nitrate. 
c = 3-66. 
Sulphate. 
c = 2-00. 
•684 
•34 
7*9 
3*7 
•48 
•12 
08 
•794 
•00 
•14 
•07 
•14 
•02 
•01 
•980 
•07 
•07 
•07 
•28 
•03 
•01 
1-27 
*05 
•40 
•20 
•19 
0-12 
•06 
The concentrated solutions were taken in the case of the bromide, iodide, 
nitrate, and sulphate, as they gave more uniform results. 
It is evident that heating has the same effect as increasing the concen- 
tration. It increases the absorption at both ends of the region investigated, 
the change in the red being most marked in the case of the chloride and 
the other salts following in the same order as before. 
The experiments described in this paper were carried out in the 
Physical Laboratory of the University of Glasgow. 
( Issued separately July 8, 1911.) 
VOL. XXXI. 
34 
