AQUEOUS SOLUTIONS.- -ARCHIBALD. 309 



foil, not easily bent. The stems of these electrodes were fused 

 into small glass tubes, which passed through, and were sealed 

 to the ebonite cover of the cell. The electrodes were kept tirmly 

 in position by means of a rubber band, passing over the cover 

 .and around the bottom of the cell. This cell, being long, and of 

 the same diameter throughout, could, by varying the distance 

 between the electrodes, be used for solutions extending through 

 a wide range of dilution. 



In preparing the double Sulphate of Copper and Potassium, 

 solutions of each salt were prepared of equal molecular concen- 

 tration. In the case of the Potassium Sulphate these solutions 

 were prepared by adding a known weight of anhydrous salt 

 (which had been dried to constant weight in an air bath) to 

 water so as to form a solution of known volume. For the Cop- 

 per Sulphate a solution was made up, analysed by determining 

 the sulphur present, and the required concentration obtained by 

 adding a known volume of water to a known volume of solution. 

 Equal volumes of these solutions were then mixed, and the 

 mixture evaporated at a temperature below 70C. If the tem- 

 perature was allowed to rise above this point, a light green 

 substance was precipitated out, which, according to Brunner,* 

 is a basic double salt of Copper and Potassium. 



When a sufficient quantity of the double salt had been pre- 

 pared, portions were weighed out and analysed for the purpose 

 of ascertaining the composition of the crystals. The method 

 adopted was the determination of the copper present by precipi- 

 tating it in the metallic state by means of pure zinc and hydro- 

 chloric acid in a platinum crucible. The results of three deter- 

 minations agreed to within 0.11 per cent and indicated crystals 

 of the composition CuK 2 (SO 4 ) 2 + 6 H 2 0. 



A solution of the double salt was then prepared and the con- 

 centration estimated by determination both of the copper and of 

 the sulphur present in a definite volume of the solution, the 

 results from the two methods agreeing to within 0.12 percent. 



* Pogg. Ann., 50, 43. 



