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 laicj 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. 



