116 
IOWA ACADEMY OF SCIENCES. 
SERIES VIII. 
NO. OF 
EXAMINATIONS. 
TIME. 
ZN REMAINING IN 
SOLUTION. 
FREE H2 SO 4 . 
1 
1 
4.24 
U 95 
2 
2 
3.68 
1.76 
3 
3 
3.35 
2.28 
4 
4 
2.95 
2.88 
5 
5i 
2.54 
3 49 
6 
2.35 
3.77 
7 
n 
2 08 
4.18 
8 
9 
1.80 
4 61 
9 
10 * 
1.37 
5.25 
The next solution experimented upon was one of cad- 
mium chloride, which contained 9.47 per cent of the metal 
and a total of 11.38 per cent 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. 
SERIES IX. 
NO. OF 
EXAMINATIONS. 
TIME. 
CD. REMAINING IN 
SOLUTION . 
FREE Hcl, 
1 
2 
4.00 
'8.72 
2 
3 
2 96 
9 40 
3 
4 
2 42 
9 94 
4 
5 
1 81 
10 15 
5 
7 
1.75 
10.19 
6 
8 
1 64 
10 26 
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 character 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 
