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 l/O.l*^ 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 3 are the solutions 

 nearer the Kipp's apparatus and 2 and 4 are their dupli- 

 cates. The solution used was nearly neutral zinc chloride: 



The fact that the duplicates contain much more zinc 

 than 1 and 3 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 w^ere placed aside 

 until it could be determined whether it was practicable to 



