116 



IOWA ACADEMY OF SCIENCES. 



SKRIES VIII. 



The next solution experimented upon was one of cad- 

 mium chloride, which contained 9.47 per cent of the metal 

 and a total of 11.33 percent of hydrochloric acid. Cad- 

 mium is so similar to zinc that the results could be pre- 

 dicted with reasonable certainty. The following table 

 gives the data from which it may be seen that cadmium 

 sulphide was still being slowly precipitated at the end of 

 eight hours from a solution containing more than ten per 

 cent of free acid. 



In summarizing the results several facts are to be noted. 

 The reactions studied are surprisingly slow, whereas most 

 heavy metals are immediately precipitated by hydrogen 

 sulphide. The precipitation curve, as one would expect, 

 slants rapidly at first, but after passing the gas through 

 the solution two or three hours the curve assumes a direc- 

 tion more and more nearly parallel with the axis of 

 abcissa. The characte* of the curve, moreover, is inde- 

 pendent of the character of the acid. (See Figure 2.) 



Agitation hastens the precipitation only very slightly, 

 and it may be assumed that it does not alter the point of 

 equilibrium. 



A moderate rise in temperature retards the reaction of 

 hydrogen sulphide and zinc only slightly, and probably 



