207 



and the water formed is entirely relained in llie j)lios|)liorns penloxide 

 tnbe /;. Wlien tiie increiu'y in ƒ and // is again at the same level, 

 it ma}^ be assnmed that everj'thing is filled with hydrogen of one 

 atmosphere. Now tlie eoek / is again closed ; k and the other tnhes 

 are again evacnated. An idea abont the pnrity of the gas whieh 

 was found in the tubes after the first filling, is now given by the 

 discharges in the cathode ray tnbe e. As long as traces of oxygen 

 still contaminate the hydrogen (i.e. with air — and the presence 

 of the latter appears at the same time — the only possible con- 

 tamination) the tube will be filled with red light. When pure hydrogen 

 has filled the tubes, the light will exhibit the well-known rice-colour. 

 An opinion may then be formed at the same time abont the degree 

 of rarefaction attained, and also about the closure of different cocks 

 and conplings. 



The tubes are then again filled with purified hydrogen, and after 

 another evacuation and filling the purity of (he gas in all the tubes 

 may be safely assumed to be sufficient. Then the cock D is closed, 

 the mercury bulb (see Conim. N". 6 p. 828) is raised, and the pure 

 hydrogen is in the first "pressure stage" ^see p. 823). Now for a control 

 the cock E was always closed once more, and the part of the tubes 

 k, a, (J, d, ƒ, and e was exhausted, to ascertain the purity of the 

 gas with which we are going to work by means of the colour of 

 the discharge light. 



If, what need not yet be done, (p. 830) it is desired first to 

 determine the normal volume of the gas at -±: 1 atm., before it is 

 brought into the first "pressure stage", F must be closed before the 

 last filling. The difference in height between the mercury levels in 

 / and i, and also the temperature of the thermostat must be deter- 

 mined. The barometric height can be read with the siphon baro- 

 meter ƒ, (/. ^) But now the tube / must be in communication Avith 

 the outer air. This may be effected by turning the cocks .1 and B 

 90° in positive resp. negative direction (Position II). Then the tube / 

 gets into communication with the outer air via Z^' and A. This remains 

 the case with ƒ/. But to prevent the tubes d and ƒ to get commu- 

 nication with the outer air also the cock A must be turned, but in 

 opposite direction. The determination of the normal volume in F has, 

 however, as said before, only sense when we wish to convey a 

 quantity of gas quantitatively to the measuring tube, which lias not 

 yet been done. 



To ascertain whether sufficiently j>ure hydrogen was obtained in 



1) By means of a cathelometer and the scalar divisions etched on the tubes. 



