112 
IOWA ACADEMY OF SCIENCES. 
a sulphide which was perfectly white. The cadmium was 
from the same manufacturer and gave a pure, bright yellow 
precipitate in acid solution. 
The solutions to be treated with hydrogen sulphide were 
placed in tubes holding about 100 c.c., immersed in a large 
water bath kept at constant temperature by means of a 
thermostat, and a stirrer driven by a small hot air motor. 
At first a thermometer graduated in tenths was placed in 
the solution and the temperature kept within 1/0.1° of 
the desired point, but this was discontinued when it was 
found how small is the influence of temperature upon 
the rate of precipitation. At temperatures above that of 
the laboratory the hydrogen sulphide was first passed 
through a tube immersed in the bath and containing 
water, in order to compensate for any loss by evaporation. 
The hydrogen sulphide was generated in a Kipp's appa- 
ratus and washed by passing it through water. The rate 
of the gas was about two bubbles per second, or about 
three to four liters per hour. Where not otherwise stated 
the temperature of precipitation was 20°. 
At first the thought was to make the determinations in 
duplicate and to this end the gas was passed through two 
solution tubes connected tandem. The following table 
shows the results. Numbers 1 and 8 are the solutions 
nearer the Kipp’s apparatus and 2 and 4 are their dupli- 
cates. The solution used was nearly neutral zinc chloride: 
SERIES I. 
NO. OF 
EXAMINATIONS. 
TIME— HOURS. 
% ZINC IN 
SOLUTION. 
% FREE ACID IN 
SOLUTION 
1 
8 
1.454 
3.24 
2 
8 
1.736 
2.92 
8 
3 
1.503 
3.18 
4 
3 
1.750 
2.86 
The fact that the duplicates contain much more zinc 
than 1 and 8 led at once to the conclusion that precipi- 
tation was by no means completed in three hours, even 
though the gas had actively bubbled through the solutions 
during the entire time. This came somewhat as a sur- 
prise, and naturally all other objects were placed aside 
until it could be determined whether it was practicable to 
