118 
IOWA ACADEMY OF SCIENCES. 
however, in such a complex system so many unknown or 
unmeasurable influences that speculation seems hardly 
justifiable at this stage of the work. 
It is hoped that the foundations have been laid for the 
more accurate separation of zinc and cadmium through 
hydrogen sulphide. Every teacher of Chemistry knows how 
often in analytical work zinc is precipitated with the metals 
of the copper group and lost, and how often cadmium fails to 
come down in its proper place in that group. From the data 
given above it is evident that the long continued action of 
hydrogen sulphide will precipitate zinc from a solution 
containing less than about four per cent of free hydro- 
chloric acid. It is also evident that cadmium will not be 
completely precipitated within a reasonable length of time 
if the solution contains more than about eight per cent of 
the same free acid. This leaves a working latitude of only 
about four per cent of free acid, and the difference becomes 
practically even less when we take into account the acid 
set free in the reaction. 
The exact conditions necessary to effect the most nearly 
complete separation of zinc and cadmium at a single pre- 
cipitation will receive further study. 
I wish here to take the opportunity to express my sin- 
cere thanks to Dr. W. S. Hendrixson at whose suggestion 
this work was begun, and to whose kindly aid and advice 
is largely due any success which this little study may have 
attained. 
