1892.] Relative Densities of Hydrogen and Oxygen. 455 



two minutes sufficed for the establishment of equilibrium, but there 

 was always a possibility of a partial obstruction. In the results to be 

 presently given four minutes were allowed after the separation from 

 the generator. It may be remarked that a part of any minute error 

 that may arise from this source will be eliminated in the comparison 

 with oxygen, which was collected under like conditions. 



The reading of the barometers and thermometers at the moment 

 when the tap of the globe was turned off took place as described in 

 the former paper. The arrangements for the weighings were also' 

 the same. 



In the evacuations the process was always continued until, as 

 tested by the gauge of the Toppler after at least 'a quarter of an 

 hour's standing, the residue could be neglected. Here, again, any 

 minute error would be eliminated in the comparison of the two 



In the case of oxygen, the errors due to contamination (even with 

 hydrogen) are very much diminished, and similar errors of weighing 

 tell very much less upon the proportional agreement of the final 

 numbers. A comparison of the actual results with the two kinds of 

 gas does not, however, show so great an advantage on the side of the 

 oxygen as might have been expected. The inference appears to be 

 that the individual results are somewhat largely affected by tempera- 

 ture errors. Two thermometers were, indeed, used (on opposite 

 sides) within the wooden box by which the globe is surrounded, and 

 they could easily be read to within gV C. But in other respects, the 

 circumstances were unfavourable in consequence of the presence in 

 the same room of the furnace necessary to heat the copper. An 

 error of +0'1 C. in the temperature leads to a discrepancy of 1 part 

 in 1500 in the final numbers. Some further elaboration of the 

 screening arrangements actually employed would have been an im- 

 provement, but inasmuch as the circumstances were precisely tlie 

 same for the two gases, no systematic error can here arise. The 

 thermometers were, of course, the same in the two cases. 



The experiments are grouped in five sets, two for oxygen and three 

 for hydrogen. In each set the work was usually continued until the 

 tap of the globe required re-greasing, or until, owing to a breakage or 

 to some other accident, operations had to be suspended. 



TOL. L. 



