857 
the compensation switch described by Morr, has been omitted‘). 
The variation of the turbidity with the time may be read from the 
registered curve. 
The use of the extinction meter of Morr for the estimation of 
the silver-content of a solution’) is based on the fact that it becomes 
turbid when a definite quantity of a sodium-chloride solution of a 
given strength is added to it. The process that then takes place, 
consists of two parts. First of all the silver and sodium ions join 
to molecules of silver chloride, which takes place very quickly, and 
then the molecular disperse solution which can be compared with a 
finely divided sol, begins to flake slowly, and consequently becomes 
turbid. After a considerable time the turbidity becomes constant. 
Hence a method worked out with a view to the study of the 
process of flaking, can also render good services for the investigation. 
For liquids with equal silver content this turbidity is only dependent 
on the time. When we, however, start from liquids with different 
silver content, it appears from the shape of the registered curve 
that this turbidity depends in a great degree on the silver concen- 
tration. We have here, therefore, an accurate means to determine 
the content of a silver solution. This method has been applied in 
the following way: 
The registered deviation of the galvanometer is plotted as function 
of the time and the area described in ten minutes*) — called area 
of flaking — is caleulated. This is done for different solutions of 
known silver content. When we now plot the calculated areas 
against the resp. silver contents, we get a gauging curve‘), from 
which the content of a silver solution that is to be examined, can 
be read after calculation of the area of flaking. 
When the sketched method is applied unmodified to the photo- 
graphic plate, we are confronted by the difficulty that the grains of 
silver lie embedded in gelatine. When the silver of the plate is 
dissolved in nitric acid, and when sodium chloride is added, the 
gelatine or its reaction products appear to prevent the flaking for 
the greater part. By varying all the circumstances and registering 
1) Versl. Kon. Akad. v. Wetensch. XXVIII 1920, p. 1002—1003. 
*) In our investigation this is always a silver nitrate solution. 
35) In general the deviation of the galvanometer has already got near its 
maximum after 10 minutes. 
*) Instead of the area of flaking also the deviation reached after then minutes 
might be used, but this method of procedure is less accurate. For the calculation 
of the area of flaking comes to the same thing as the use of different deviations 
separated from each other by equal time intervals. 
56 
Proceedings Royal Acad. Amsterdam. Vol. XXIII. 
