IOWA ACADEMY OF SCIENCES 
111 
If we call the original concentration of zinc C, and 
start with a neutral solution the equivalents of ZnS pre- 
cipitated or the HC1 set free at any moment X, — and the 
specific rates of the reactions from left to right and from 
right to left, respectively, k and k„ then the rates at any 
moment will be (C - K)h.k and Iv 2 .k' and equilibrium will 
be reached when (C - X)h.k = X 2 .k', where h represents the 
hydrogen sulphide wdiich is regarded here as constant. 
Before proceeding with the presentation of the data, a 
few T w T ords regarding the quantitative methods used are 
desirable. It was necessary to determine the total acid in 
each solution at the beginning, and the metal remaining 
in the solution at the end of any chosen period. From 
these data the free acid at the end of any period could be 
calculated. Sulphuric acid was determined as barium 
sulphate in the usual way, hydrochloric acid volumetric- 
ally by the method of Volhard, cadmium as the sulphide 
which w^as weighed in a Gooch crucible, and zinc by the 
ferrocyanide method of Lyte. 
The last named method seems not to have attained a 
use to which its accuracy and rapidity entitles it, and a 
few words in recommendation of it are not deemed inap- 
propriate. The solution is acidified with hydrochloric 
acid and titrated with a solution of potassium ferrocyanide 
which has been standardized by a zinc solution of known 
strength, the titrations being carried on at about 70°. 
Uranium acetate is used as an indicator. A few drops 
may be added to the solution which is colored brown when 
the ferrocyanide is added in slight excess. It is more 
exact to bring a drop of the solution under titration and 
near the end point, in contact w T ith a drop of the indicator 
on filter paper. A brown line is formed where the two 
drops flow together. With care titrations are concordant 
to a few hundredths of a per cent., and the method seems 
to deserve rank among the standard methods of volumetric 
analysis. 
The zinc used in this work was a pure distilled specimen 
from Schuchardt, which dissolved without residue and gave 
