256 PROCEEDINGS OP THE AMERICAN ACADEMY 



used, the method being essentially that of the first experiment. The 

 large crucible of one hundred and fifty cubic centimeters' capacity 

 used for the electrolysis was not previously coated with copper. The 

 end point of the acidimetric determination was taken with both phenol 

 phthalein and methyl orange, the color change with the latter indicator 

 being rather unsatisfactory in the presence of so large an amount of 

 dissolved substance. The amount of insoluble residue found in the 

 sodic sulphate was only 0.0003 gram; it contained no trace of copper. 

 The results showed that little was to be gained by the use of larger 

 quantities of substance, since the unavoidable errors of quantitative 

 work were multiplied nearly in proportion to the quantities of mate- 

 rial. Mechanical errors are at the present day inessential compared 

 with the constant ones which complicate so many analyses. It is 

 obvious that a far more accurate result could be obtained from four 

 tenths of a gram of really anhydrous cupric sulphate, than from four 

 hundred grams of a substance still retaining nearly two tenths of a per 

 cent of water. 



Adding in the small amount of water which is lost by cupric sul- 

 phate between 255° and 365", the percentage composition of the 

 crystallized salt, as indicated by the second series of analyses, gives 

 a much more satisfactory total result than before : — 



Water lost at 255° = 35.960 



Additional water lost at 365° = 0.108 



Copper =: 25.450 



Sulphuric acid radical = 38.436 



Total = 99.954 



Although greatly improved, the analysis still leaves much to be 

 desired. Even in materials prepared with the described precautions, 

 traces of impurity were manifest. In the last experiment, where a 

 large amount of material was used, a few minute spots, which might 

 have been arsenic, appeared on the clear surface of the electrolytic 

 copper, and traces of a brownish coloration were observed upon the 

 positive pole. Both of these impurities were so infinitesimal in 

 amount that they could not reasonal)ly have been expected seriously 

 to affect the final result; but the thought that they might be responsi- 

 ble for a part of the remaining deficiency prompted the execution of a 

 still more elaborate series of experiments. The data and results of 

 the second series are appended : — 



