92 THE ATOMIC WEIGHTS. 



In 1873 Hampe* published his careful determinations, which were 

 for many years almost unqualifiedly accepted. First, he attempted to 

 estimate the atomic weight of copper by the quantity of silver which 

 the pure metal could precipitate from its solutions. This attempt failed 

 to give satisfactory results, and he fell back upon the old method of 

 reducing the oxide. From ten to twenty grammes of material were 

 taken in each experiment, and the weights were reduced to a vacuum 

 standard : 



20.3260 grm. CuO gave 16.2279 grm. Cu. 79.838 P er cent. 



20.68851 " 16.51669 " 79.835 " 



10.10793 " 8.06926 " 79-831 " 



Mean, 79.8347, rfc .0013 



Hampe also determined the quantity of copper in the anhydrous sul- 

 phate, CuS0 4 . From 40 to 45 grammes of the salt were taken at a time, 

 the metal was thrown down by electrolysis, and the weights were all 

 corrected. I subjoin the results : 



40.40300 grm. CuSO 4 gave 16.04958 grm. Cu. 39.724 per cent. 

 44.64280 " 17.73466 " 39-7 2 6 



Mean, 39.725, .0007 



The last series of data gives Cu =62.839, .0035, and is interesting 

 for comparison with results obtained by Richards later. 



In all of the foregoing experiments with copper oxide, that compound 

 was obtained by ignition of the basic nitrate. But, as was shown in the 

 chapter upon oxygen, copper oxide so prepared always carries occluded 

 gases, which are not wholly expelled by heat. This point was thoroughly 

 worked up by Richards f in his fourth memoir upon the atomic weight 

 of copper, and it vitiates all the determinations previously made by this 

 method. 



By a series of experiments with copper oxide ignited at varying tem- 

 peratures, and with different degrees of heat during the process of reduc- 

 tion, Richards obtained values for Cu ranging from 63.20 to 63.62, when 

 , O = 16. In two cases selected from this series he measured the amount 

 of gaseous impurity, and corrected the results previously obtained. The 

 results were as follows, with vacuum standards : 



1.06253 g rm CuO gave. .84831 grm. Cu. 79.802 per cent. 



1.91656 " 1.5298 " 79.820 " 



Mean, 79.811, .0061 



Correcting for the occluded gases in the oxide, the sum of the two 

 experiments gives 79.901 per cent, of copper, whence Cu = 63.605. Three 



* Fresenius' Zeitschrift, 13, 352. 

 fProc. Amer. Acad., 26, 276. 1891. 



