20 THE ATOMIC WEIGHTS. 



up again to 15.865. Still another correction is suggested, namely, that 

 as the reduced copper in the combustion tube, before weighing, was ex- 

 posed to a long-continued current of dry air, it may have taken up traces 

 of oxygen chemically, thereby increasing its weight. As this correction, 

 however, is quantitatively uncertain, it may be neglected here, and the 

 result of this series will be taken as = 15.865, .0103. Its weight, 

 relatively to some other series of experiments, is evidently small. 



In their second and final series Dittmar and Henderson dried their 

 hydrogen, after deoxidation by red-hot copper, over caustic potash and 

 subsequently phosphorus pentoxide. The copper oxide and copper of 

 the combustion tube were both weighed in vacuo. The results were as 

 follows, vacuum weights being given : 



Wt. Water. Wt. O. At. Wt. O. 



19.2057 17-0530 15.843 



19-5211 17-3342 [15-853] 



19.4672 17.2882 15.868 



22.9272 20.3540 15.820 



23.0080 20.4421 [15.934] 



23.4951 26.8639 15.859 



23.5612 20.9226 [15-859] 



23.7542 ^ 21.0957 15.870 



23.6568 21.8994 15.884 



23.6179 21.8593 15.848 



24.6021 21.8499 15.878 



24.3047 21.5788 15.832 



23.6172 20.9709 15.849 



Mean, 15.861,^.0052. 



The authors reject the three bracketed determinations, because of 

 irregularities in the course of the experiments. The mean of the ten 

 remaining determinations is 15.855, .0044. Both means, however, 

 have to be corrected for the minute trace of hydrogen occluded by the 

 reduced copper. This correction, experimentally measured, amounts to 

 -|- .006. Hence the mean of all the experiments in the series becomes 

 15.867, .0052, and of the ten accepted experiments, 15.861, .0044. 

 The authors themselves select out seven experiments, giving a corrected 

 mean of 15.866, which they regard as the best value. Taking all their 

 evidence, their two series combine thus : 



First series 15.865, .0103 



Second series 15.867, .0052 



General mean 15.8667, .0046 



Leduc,* who also effected the synthesis of water over copper oxide, 



* Compt. Rend., 115, 41. 1892. 



