( 509 ) 



tube V^i- Two cases are (o he disliniïnislied here. Willi oiteralions 

 at ordinary ))ressiire it will be siifiicieiil lo adjust the eryostat at 

 a jn-essure a little liiglier than that cd' the atmosphere and to either 

 connect the cock V^^ with a fj^asiiolder (rtt:. or lo disconnect them, 

 as the occasion demands. As soon as the pressure passes a certain 

 limit settled foi- the eryostat, the gas escaj)es from the eryostat 

 through the large safety api)aratus. For op(M-ations at reduced pres- 

 sure, the eryostat, after the [)ressure has been sufficiently lowered 

 by means of the exhaustpump of the circulation Krh. 1, is discon- 

 nected from the latter and connected by means of the cock F^, to 

 the exliaustpump E.i'h. 2., and is then reduced to a lower pressure. 

 Obviously we can sometimes avail ourselves for this latter operation 

 of the same exhaustpump as used with I'J.rh. 1. The evaporation 

 will proceed more gradually when a connection is made lo a reservoir 

 at i-educed pressure Vac, plate. If a reser\'oir of large volume is 

 used we can even work without an exhaustpump, which may be 

 valuable when it is necessary to avoid vibration for the measure- 

 ments. Thus with the bath of nitrogen under diminished pressure 

 the auxiliary compressor of Comm. 54 .Ian. '00 plate VII was 

 connected near Kr/i. I to the gaslead and the vessel of 5 m' men- 

 tioned above (comp. (^omm. 14 Dec. '1)4 § 10) served as vacuum 

 reservoir, after being exhausted through }\g and V^^ by means of 

 a BuKCKHAiiUT \'acunmpump, connected to the gaslead at Kc/t. 2. 

 This vacuumpump will be described later. 



In a few words we shall indicate the method which we usually 

 follow in order to get a welllille<l bath at diminished-pressure. 

 First the double beaker 7>„i, />*„„, or several beakers />\, I)^, />\ 

 are tilled at ordiiuiry pressure, then we begin to slowly exhaust 

 through )\^; all other cocks being shut by means of the pump, 

 generally use<l for the circulation /'J.r/i.. 1; while boiling is prevented 

 by rapidly moving the stirring apparatus described in § -i- When 

 the recpiired ])rcssure is reached the eryostat is to be conncctcMl to 

 the great reservoir I'^ac. at the same |)ressin'('. If this caiuiol be done 

 we hardly ever succeed in admitting through the cock A„ the yet recpiired 

 (piantity of licpiid slowly enough to keep the pressure in the eryostat 

 free from undesirably large fluctuations or even lo avoid with the 

 liel|> of )\,., momentarily returning of it to nearly its ordinary value. 

 Therefore, if a change of tenii)eratnre foi- some time is alloNNcd, it is in 

 that case better to shut }\^ before more li<piid is added ami lo connect 

 the eryostat through 1",,- to the gasholder. As long as the beaker 7J„, 

 is not full the gas leaving the eryostat is allowed to pass through 

 }",5 into the gasholdei'. If the beaker />V^ is full, which is shown by 



