f 500 ) 



(*omni. 51, as iiiucli as |M>s,sihlc in ilio aiTangenicnl of llie measuring 

 apparatus considered liere. Foruiorly the halli conhl be excentrieallv 

 luounlod wKli relercnce lo llie tube J'\ wlioi-eas liiis time a central 

 inouiitini!; was desirable. The existing- dimensions of |)arts of the 

 apparatus have also had the result that in the experiments described 

 hei'e the bath must be placed a little too high with regard to the 

 observing glasses V.^, which might easily have been avoided if we 

 iiad beei] perfectly free in our construction. 



The glass ring 11^, not occnri-ing in the arrangement of Comin. 51, 

 serves still better to screen the bath from external heat. T>ike the 

 other beakers and glass cylinders B^, />\, /i,, B^, ƒ>'„,, B^^, it is 

 silvered inside and outside, leaving open, however, vertical strips 

 nearly corresponding in width with the resistajicc thei'mometer y>. 



The conical rim B^^ lies loose on the beaker B,,^. When the liquid 

 boils up, it streams back to />„ along the wall of the funnel; if, 

 however, ^„^ is filled to the brim and more licpiid is poured in, 

 this superfluous liquid flows over into the beaker /:>„.j, which also is 

 filled before a measurement is made. If an intense cooling of the 

 neighbourhood of the l>ath is requii-ed, the beakei's />j, /:>^, />, must 

 also be filled. It should be remembered, however, that if this is 

 done, the evaporation at low pressure, as long as liquid remains 

 in the outer beakers, requires a |,)0werful vacuunq)ump. 



The l)atii itself oidy evaporates slowly. Instead of the double 

 beakers 7i„i Bf,,^ we might take a vacuum glass in onler to diminish 

 the evaporation as has sometimes been done (conqi. § 3). But it is 

 ji(»l always easy to obtain vacuum glasses of the recpiired dimensions ajid 

 internally finished with the accuracy necessary for the j)ro|)er working 

 of the stirring apj)aratus. Moreover one will iiol b<' inclined t<» 

 iiiiincrso delicate measuring apparatus in the l>ath before oiieissulli- 

 cieiitly cei'tain that the vacuiun glass will iioi bui-st as such ofgreater 

 (liniensions sometimes do. 



§ 3. To make clear the i)ui'pose of the arrangements to be described 

 in the jiext sections, it seems lo me that the following particularisa- 

 tions will be useful. First of all the temperature gradient in the 

 bath. Even when the li(piid boils regularly we find that in the lower 

 layers, as a result of the hydrostatic pressure, the lenq)erature exceeds 

 that of the upper layers. If, as often happens with greatly (bminished 

 ))ressures when boiling is not produced artificially, cndy evap(n*atioii 

 at the surface occurs instead of boiling, the tenq)erature in the upper 

 lavers of the bath may fall considerably below that of the lower. 

 If then the li(iiiid siid<leiiU boils up, which always ha[»[)ens whenever 



34 



Proceedings Royal Acad. Amsterdam, Vol. V, 



