OXYGEN. 15 



careful series of measurements they found that the correction amounted 

 to an average increase of 1.98 milligrammes to the weight of hj^drogen 

 taken in each experiment. Hence equals not 15.953, but 15.869, the 

 probable error remaining unchanged. The final result of Cooke and 

 Richards' investigation, therefore, is 



O = 15.869, ± .0020. 



Reiser's determinations of the atomic weight of oxygen were published 

 almost simultaneously with Cooke and Richards'. He burned hydrogen 

 occluded by palladium, and weighed the water so formed. In a pre- 

 liminary paper* the following results are given: 



Mean, 15.864, =b .015 



Not long after the publication of the foregoing data Reiser's full paper 

 appeared.f Palladium foil, warmed to a temperature of 250°, was satu- 

 rated with hydrogen prepared from dilute sulphuric acid and zinc free 

 from arsenic. From 100 to 140 grammes of palladium were taken, and 

 it was first proved that the metal did not absorb other gases which might 

 contaminate the hydrogen. Before charging, the foil was heated to bright 

 redness in vacuo. After charging, the tube containing the palladium 

 hydride was exhausted by means of a Geissler pump to remove au}^ 

 nitrogen which might have been present. In the preliminary investiga- 

 tion cited above, the latter precaution was neglected, which may account 

 for the low results. 



Between the palladium tube and the corhbustion tube a U tube was 

 interposed, containing phosphorus pentoxide. This was to determine 

 the amount of moisture in the hydrogen. The combustion tube was 

 filled with granular copper oxide, prepared by reducing the commercial 

 oxide in hydrogen, heating the metal so obtained to bright redness in a 

 vacuum, and then reoxidizing with pure oxygen. 



Upon warming the palladium tube, which was first carefully weighed, 

 hydrogen was given off and allowed to pass into the combustion tube. 

 When the greater part of it had been burned, the tube was cut off by 

 means of a stopcock and allowed to cool. Meanwhile a stream of nitro- 

 gen was passed through the combustion tube, sweeping hydrogen beford 

 it. This was followed by a current of oxygen, reoxidizing the reduced 

 copper; -and the copper oxide was finally cooled in a stream of dry air. 

 The water produced by the combustion was collected in a weighed bulb 

 tube, followed by a weighed U tube containing phosphorus pentoxide. 



* Berichte, 20, 2323. 1887. 



t Atner. Chem. Journ., 10, 249. 188S. 



