244 PROCEEDINGS OP THE AMERICAN ACADEMY 



The third aud fifth of these comparisons were made with the bal- 

 ance in the condition of greatest sensibility, while the others were 

 made exactly in the usual manner. Jt is not pretended, however, 

 that all the weighings which follow have probable errors so small as 

 these. The results are nevertheless an excellent gauge of efficiency 

 of the balance, especially since the observations were made at widely 

 varying temperatures and pressures. The correction involved, apply- 

 ing in the same proportion to all weighings, has been omitted from the 

 following figures; but any weight given below may of course readily 

 be reduced to the Washington standard by multiplication with the 

 factor 1.000025. 



I. The Analysis of Cupric Sulphate. 



The only published analyses of cupric sulphate which have had for 

 their object the determination of the atomic weight of copper are those 

 previously mentioned. Hampe has very clearly described two ex- 

 tremely concordant electrolytic determinations of the copper contained 

 in the so-called anhydrous salt. The greatest care had been taken to 

 separate minute traces of metallic impurities from the preparation, 

 which had been dried at 250° Centigrade ; but apparently it was not 

 realized that for the purpose in hand the retention of a trace of water 

 by the salt was a much more serious possible cause of error than 

 any other. Baubigny's analyses are less fully described, so that it is 

 less easy to judge of their value. A different method, the conversion 

 of cupric sulphate into the oxide by heat, was adopted for these ex- 

 periments. The result was very different from that obtained by 

 Hampe, but was more nearly accurate, because complicated with a less 

 serious combination of constant errors. 



The entire reconciliation of all these results, not only with each 

 other, but also with the more recent analyses, involved the complete 

 analysis and synthesis of cupric sulphate. A somewhat detailed ac- 

 count of the various operations is given below. Many of the less 

 important points are necessarily omitted on account of the already 

 too great accumulation of material, and many relations between the 

 figures can be worked out by those whom they may interest. It is to 

 be hoped that the effort to spare the overburdened literature of to-day 

 any unnecessary additions may not have interfered with the clearness 

 of the description. The first three series of experiments upon the 

 analysis of cupric sulphate constituted a study of the effect of pro- 

 gressive refinement, and the first of the three perhaps hardly deserves 

 a place in a paper upon atomic weights. Unless otherwise stated, the 

 (lata are given in full. 



