PAR']' 111.— ON THE VOLUMETRIC COMPOSITION OK WATER. 



1 . INTRO^>TI(!TIO^f. 



A kiiowleilgi- (if the density of oxygen and hydrogen is iniportant, even with- 

 out reference to the fnrther (juestion of atomic weight. But my |iiinci[)al object 

 was to use these determinations to deteiinine tlie hittei'. Eor this purpose, it is 

 necessary to know also the ratio of tlie volumes in which the two gases combine to 

 form water, the measui'ement to be effected in conditions nearly the same as those 

 in which the volume and pressure were measuretl in the determination of density. 

 This ratio I have attempted to nu\asui'e by eudiometi'ic methods. My apparatus 

 for tli<; purpose was made about fourteen years ago, but it was never used with 

 oxygen and hydi'ogen till the measurements which were published in 1891.* Before 

 this, the manipulation of the ap|iaratus had lieen mastered in a long series of 

 analyses of air. The erroi's of measurement of a volume of gas with this apparatus 

 de])end on the errors of deteiniiuing tem[)erature, and on the error in I'eading the 

 level of the mercuiy in the eudiometer; for the error at the top of the mauometric 

 column of mercury is, by the use of suitable means of exact setting, made evanescent 

 in comparison with the other two errors. These two soui-ces of error cannot be 

 well separated. But a discussion of all the analyses made up to a certain date 

 showed that if the probable error of volume in measuring a gas be attributed to 

 either one of these sources, supposing for the moment that the other error did not 

 exist, then the pi'obable error of level was less than the hundi-edth of a millimeti'e, 

 or the probable erroi' of tempei'atui'e was less than the hundredth t)f a degree. Why 

 therefore any one should call it an objection to my measurements that I read the 

 level to the twodinndredth of a millimetre is incom|)rehensible. 



The reason why my measurements do not agree with those of Scott is not yet 

 ex[ilained. He says that the reason is to be found in tli(^ presence of ethane in ray 

 hydiogen, due to the electrolysis of imjnire hydroxide; but I never used potassium 

 or sodium hydroxides ; as my paper states, I abandoned their use during my pre- 

 liminary experiments. Cai-bon ilioxide was always sought for, and all which I found 



* Aiiicru<i/i yciir/ia/ oj Science, 41, 220. 

 8i 



