180 



case of neon, although we hope before long to be in a position to 

 carrj out this purpose. ') 



As regards the method of deducing the critical data from the 

 observations we have in oiir Communication on the critical condition 

 of neon referred to the similar case of hydrogen, which is the 

 subject of our present communication. In this case we are able to 

 illustrate our method of procedure by means of a suitable diagram '). 



This tigure gives tiie main features of the pressure-density 

 diagram, drawn during the observations to keep a graphical record 

 of the results already obtained, in order to guide us, as we went 

 along, in the choice of the conditions under which the next observa- 

 tions had to be made. 



In representing an observation we used as abscissa the quantity 

 of gas ') that was at that moment present in the part of the 

 vapour-pressure apparatus which was at the temperature of the 

 observation *). 



The observations relating to one isothermal are each time indicated 

 by one and the same of the signs D, O, x and A. The determination 

 of the points of beginning and completed condensation had been 

 preceded by tentative readings in the neighbourhood of the obser- 

 vations recorded in the diagram. The fact that the heterogeneous 



1) Compare, besides the paper already quoted on the hydrogen-vapour cryostat 

 (151a), particularly H. Kamerlingh Onnes, Proc. XVIII. pg. 507. Gomm. N". 147c, 

 where a cryostat is discussed working with neon boiHng under enhanced pressure 

 to be used amongst others for this object. 



2) The corresponding diagram of neon being somewhat modified owing to 

 impurities would necessitate an elaborate discussion. 



') The quantity is given in cc. of gas measured in the normal condition. 



*) The volume of this part is approximately l.Ot cc. Our apparatus, being a 

 simple vapour-pressure apparatus, was not suitable either for the accurate measu- 

 rement of this volume or of the quantity of substance which enters this volume. 

 In consequence of this the uncertainty as regards the accurate value of the volume 

 which is at the observational temperature as also the value of the temperature 

 of certain other parts of the apparatus whose volume cannot be neglected is 

 such that the accuracy of the density-determinations cannot be guaranteed 

 beyond a limit of a few percentages. 



Owing to the uncertainly of the correction for the "waste" space, the figure 

 may deviate in a systematic manner from the correct pressure-density diagram to 

 the same degree: in order to obtain the correct figure the chords of the boundary 

 curve i. e. the heterogeneous parts of the isolhermals will have to be shifted 

 without any change in length, each by an amount about proportional to the 

 pressure ; the uncertainty about the part of the volume which has the low obser- 

 vational temperature causes the value of the unit of density which is assumed 

 in setting off the density on the axis of abscissae in fig. 1 to be only approxi- 

 mately known. 



