C-j DENSITIKS OK OXYUKN AND HYDROGEN 



that IK) :il)S(>rptii>n would takf [tlaoe, a tMirrent was passed tlirougli it, ami this was 

 continued for half an hour; eare wjis taken to make this current pass through 

 every branch of the connecting tubes so as to expel all air. When the exit of the 

 hydrogen had been closed by fusion, the palladium was cooled, and hydrogen was 

 admitted till its tension was equal to that of the atmosphere. Ilxdrog.-ii was then 

 again passed through the tube of palladium, escaping into the air f^r three hours, 

 so as to remove any nitrogen which should have accumulated. AN'litii this had been 

 accomplished the exit of the gas and the connection with the voltametei' were then 

 closed by fusion. The tulje containing palladium had been from the first connected 

 witli the globe, but tlir jiassage iiad been stopped by a plug of fusil)le metal. 



8. SECO>'I) METUOO. lII.MNfi (iLOBES Willi IIVI)K0«iEN. 



When the glol>e and its connections had been exhausted to the desired degi-ee, 

 niu- ..I' the fusible metal plugs which ()l)structed the connection with the supply 

 of hydrogen was fused, and hydrogen admitted. When the pressure had become 

 some few centimetres, the connection with the palladium was closed by fusion, and 

 the irlobe was asrain exhausted. When the exhaustion was suliicient, a second 

 fusible metal plug was removed, and hydrogen admitted till the atmospheric pres- 

 sure was i-eached. The preliminary exhaustion of the globes was always such that 

 less than a hundred-thousandth of air remained. For the second exhaustion, a 

 vacuum of a tlinusaiidtli was thought sulticient. 



9. SKrOXD MJniloD. ^IEASIHKMENT OI' PRESSURE. 



When the irlobe had been filled with hydrotren, the ice in which it stood was 

 well heaped around it, and the cover leplaced over the tank. IJeadings of the 

 sy[thon barometer were made as in the case of oxygen. In all cases, the two levels 

 of the mercury to be read were in the same perpendicular, and the scale \ised stood 

 in contact with the glass tubes of the barometer, so as to minimize the effect of any 

 want of parallelism in the directions of the motions of the reading microscopes. 



It will be seen that in all my experiments on the density of gases, their 

 pressure has been determined without any conununication between the gas and 

 the atmosphei-e ; contamination was therefore impossible. It was necessary that 

 my work should be so .nranged that it could be left at almost any point with 

 little danger of the lo.ss of an exjieriment in conseijuence of the iiit('riii]iti()n. 



In the first series of ex[iei'iments with oxygen ami hydrogen, a inaiio-liaronieter 

 was used, owing to the conditions of the exi)eriim'nt ; but in all other ex[>eriment8 

 the two central tubes of the manometer, which together constituted a syphon 



