528 
Proceedings of the Koyal Society of Edinburgh. [Sess. 
the region investigated and is practically unaltered for intermediate values 
of A. In the red the increase is much more marked in the case of the 
chloride, then come in order of magnitude the bromide iodide and nitrate, 
and for the sulphate we have a very slight decrease. 
After finishing the effect of concentration, I commenced to investigate 
the effect of temperature. The experimental difficulties usually met with 
here will be more fully discussed in one of the later papers of this series. 
It was resolved to keep the method as simple as possible. One cell was 
taken, of the same type as before, 3x2x1 cm., inside measure ; it was filled 
with solution, and if the strength of the latter made it necessary a glass 
block was also placed inside the cell. The cell was then placed in a thin 
strip of aluminium, as illustrated in the diagram. AB is a rubber band 
Fig. 3. 
which keeps the strip tight. A copper vessel was filled with water, the cell 
placed in it with the horizontal parts of the strip resting on the edge of the 
vessel, and the upper surface of the water almost level with the top of the 
cell. The cell was heated by placing a bunsen below the copper vessel. 
Before heating, the cell was placed before the slit and the deflection noted 
twice. The cell was then heated until the temperature of the solution as, 
read by a thermometer was 80° C. It was then placed before the slit and 
the deflection noted twice, during which time the temperature usually fell 
to about 60° C. The cell was then cooled down to room temperature by 
placing it back in the heater and gradually cooling the water ; two readings 
were then taken when it was cold. It was then heated a second time, other 
two readings taken, then again cooled to room temperature and a final two 
reading's taken. 
The set of readings took an hour. 
From all these observa- 
tions s and s', the mean deflections respectively for room temperature and 
for 70° C., were calculated. If A denote the molecular extinction coefficient 
at room temperature and A' at 70° C., 
s l(k Ac£i 
10 
_ JQcd(A'-A). 
Hence 
A' - A = — log s/s', 
cd & 7 
