INTRODUCTION. 9 



was not properly evaluated, Stas believing that his work on the chloride 

 also indicated an atomic weight above 39.14. 



In view of the fact that these results differ among one another to an 

 extent beyond a reasonable limit of experimental error, it seemed advis- 

 able to investigate once more the atomic weight of potassium in order to 

 detect the cause of the discrepancy. This appeared to be especially desir- 

 able on account of the recent gain in knowledge concerning the peculiari- 

 ties of argentic chloride in solution. Accordingly, the present research 

 was begun, and simultaneously another concerning potassic bromide. A 

 description of the latter investigation follows immediately after this one. 



An investigation upon atomic weights naturally resolves itself into 

 several different portions --first, the preparation of the pure material; 

 second, the method of drying and weighing this material ; third, the 

 details of the analysis and the calculation of the results. These will be 

 considered in order in the following pages. 1 



PREPARATION OF THE MATERIAL. 

 POTASSIC CHLORIDE. 



Several methods of preparing potassic chloride were tried, the most 

 efficacious being adopted. The problem, of course, was to effect the 

 elimination of other metals and other acids, especially those of the same 

 groups. Both ends are most quickly and completely attained by incor- 

 porating into the process of preparation successive crystallization of 

 different salts, in order to eliminate by the crystallization of one salt an 

 isomorphous substance which might have been retained during the crys- 

 tallization of another salt. The commonest salts of potassium were 

 therefore studied in relation to their fitness for the elimination of impur- 

 ities in this way. 



In the first place, on account of the good results which Richards and 

 Wells had attained in preparing pure sodic chloride from recrystallized 

 sodic sulphate by precipitation with gaseous hydrochloric acid, sulphate 

 of potassium also was early considered. This method was, however, soon 

 abandoned, because the relative solubilities of potassic sulphate and 

 chloride are far less favorable for the purpose than those of the sodium 

 salts. The method of Archibald, who converted sulphate into chloride 

 through precipitation with baric chloride, is complicated and involves the 

 double work of preparing pure baric chloride as well as pure potassic 

 sulphate. Moreover, it is not easy to eliminate the last trace of baric 

 sulphate from the resulting salt. This also was rejected. 



1 A somewhat less detailed statement of this work has appeared in the Berichte d. 

 deutsch, ch. Gesel., 39, 3611 (1906). 



