64 DENSITIES OF OXYGEN AND HYDROGEN 



merouiiiil i>imips will give results uot far from the value obtaiued. l>ui lliey hy 

 no means show that this value is the trutli. If we can trust to a method which 

 seems free from objection, and whirh has l)eeii carried out with as much care as 

 can well be used, the value here obtained is not far from one thousandth i>art too 

 lai'ge. 



12. HYDKOGKN liY NKW Mirilli'H. INTKODHOTION. 



The accidental errors of the measurement of tluMlensity of hydrogen by weigh- 

 in" a c'iven volume are considerable. A globe containing 1.8 grammes of hydrogen 

 under standard conditions, and itself weighing not more than twelve hundred 

 frararaes, can be obtained, though with difficulty. The gross weight of the globe 

 is then si.\ hundred times the weight of the hydrogen to be determined by its use. 

 It is true that the weight of the globe at any given stage of the experiment can be 

 determined witli an ei'ror of not much more than one tenth of a milligramme, and 

 that in no long time, provided a pretty elaborate plant be employed. But it has 

 not yet been found easy, I believe, by any one, to secure constancy of the weight 

 of the "-lobe and of the lubrication of its stojicock dui'ing the e.xposnre to contact 

 with water, which is necessary if a constant tem})erature is m.iint.iined in the 

 usual way. 



It is possible to avoid the use of a stopcock on the globe in wliit-li a gas is 

 wei<'hed,and the manipulation foi' this purpose is not troublesome. I had expected 

 to make use of this method : l>ut a severe exjilosion broke nearly all my calil>rated 

 "lobes. It is proljable that some increase in [H't't-ision rould lir oKtaini-d liy this 

 method. For the globe, after tilling with the ga.s, and after thorough cleansing, 

 could be put in a desiccatoi', and kept on the balance till it was eertain that all 

 eflFects of contact with water had disajypeared. It could tluu he exhausted without 

 removal from the desiccator, closed b}' fusion, and again weighed. During both 

 weighing.s, the globe would have a surface of continuous gl.ass. Still, it is doubtful 

 whether the labor of making a sufficient number of determinations would be well 

 bestowed. 



For, unless elaboi-ate pi-ecantions are taken, the efficiency of which i-emains to 

 be proved, mercurial vapor may diffuse into the globe in which the gas is to be 

 weighed. I'ntil, therefore, we can dispense with the use of mercurial pumps and 

 yet jiroduce a vacuum of a few millionths, it seems necessary to devise some 

 method of measuring the density of hydi'ogen in wliich the containination of the 

 gas with the vapor of mercury shall occasion no error. 



The weighing of the hydrogen while it is absorbed in ])alladium, and 

 measuring its volume and ])ressure in another vessel after ex[)elling it from 



