( 239 ) 



naked eye with sidc-illiimiiuitiöii of llic iiiirrur proved to l)e tlie best. 

 The difierenee raiigetl betAveen limits which amounted to 27o oftiie 

 pressure. The optical part of the method ma^- certainly be much 

 improved. The accuracy attained will, however, be sulïicient in 

 many cases. 



Adiabatic ])ressure-variation must naturallj' be avoided. Yet all 

 such difficulties arise also with measurements at ordinary tempera- 

 ture. With a view to the large dimensions of the vapour space a 

 we have not made use of a stirrer and have tried as much as 

 possible to surmount the difficulties by operating slowly. 



§ 5. Determination of the end eondensation jiressvre. For this 

 determination the mixture, after the begin condensation pressure 

 is measured, is led back to the volumenometer, the dew-point 

 apparatus is disconnected from the steel capillary and in its place 

 the piezometer j) in ^ PI. 1 figs. 1 and 3 is connected with the steel 

 capillary //"j. The piezometer consists of a wide glass tube j)^ PI. I 

 fig. 3 fastened to a capillary, both graduated and calibrated. The 

 dimensions are chosen ^vitll regard to the quantities of gas that 

 may be intended for the measurements. If these are decided upon, 

 the exact quantity of gas, necessary for tilling the piezometer at a 

 suitable position of the mercury in the pressure tube h with liquid 

 to near the end of the capillary, must be determined before each 

 determination by means of a preliminary experiment. 



The equilibrium of the phases in />. is reached by means of a 

 magnetic stirrer q nioxed by the coil X The immediate effect of 

 this coil is not sufficient to move the stirrer forcibly through the 

 liquid meniscus. Therefore a soft iron tube z^ with a groove z.^ which 

 enables us to read on p^,, is moved \\\) and down at the same time 

 Avitli the coil. 



This movement ought to be independent from that of the stirrer 

 in the cryostat. But as we did not require a \qy\ high degree of 

 accuracy in our experiments we liaxe for simplicity devised the iron 

 tube z as a conuection between the upper aud the lowei' pai't Xn» 

 and Xn4 <^f ''iG ringshapcd valved stirrer (comp. Comm. N". 83). 

 This is moved up and down mechanically and with the hand in 

 turns, one time to stir the liquid bath, the other time to establish 

 equilibrium between the li([uid and the vapour while we simultane- 

 ously move the magnetizing coil S. 



The essential dilference between a determination of the end con- 

 densation i>ressure in our apparatus and that in a (Uillktkt tube 

 does not lie so much in the circumstance that we do not licpiefy 

 the whole (luantity of gas, as in the fact that, as remarked in 



