398 Seientific Proceedings^ Royal Dublin Society. 



At temperatures below the boiling-point the compressibility is so slight 

 that the determinations may be made without sensible error under atmo- 

 spheric pressure. Above the boiling-points the pressure should be made as 

 nearly as possible equal to the vapour-pressure. 



No difficulty is experienced until the temperature reaches about 0'85 of 

 the critical temperature (calculated on the absolute scale), because a small 

 alteration of pressure has ho sensible effect on the volume. At higher 

 temperature, however, the liquid becomes so compressible that it would 

 be necessary to make the pressure exactly equal to the vapour-pressure ; but 

 it is difficult to do this, because at these relatively very high temperatures 

 ebullition takes place with the utmost ease, even when the liquid is quite free 

 from air. If, when the vapour is all condensed, the pressure is reduced even 

 slightly below the vapour-pressure, ebullition takes places at once ; and the 

 pressure must again be raised to condense the vapour formed. If, on the 

 other hand, the pressure is even slightly too high, the observed volume of 

 liquid will be appreciably less than the orthobaric volume. 



In order to obtain accurate results, a method' was in most cases adopted 

 which has also been described and employed by Mathias^ and by Amagat.^ 



Starting with the vapour completely condensed, the pressure is reduced 

 until ebullition takes place and a little vapour is formed. When the pressure 

 is quite steady, readings of the volumes of liquid and saturated vapour 

 are taken. The volume is then increased ; and when the pressure has again 

 become constant, the volumes of liquid and saturated vapour are again read. 

 The process is continued so as to obtain four readings of each of the two 

 volumes. Each time that the volume is increased some of the liquid evapo- 

 rates so tliat the volume of liquid diminishes and that of the saturated 

 vapour increases. 



Calling the first and third volumes of liquid vi and v"' and the 

 corresponding volumes of saturated vapour «'/ and i\"', the ratio R of the 

 specific volume of saturated vapour to that of liquid is given by the 

 equation 



%'" - v/ v,"" - v" 



A = — ; m = —Tr 777? (o) 



In this way two independent values of i? are obtained ; and the mean, if 

 the agreement is good, may be taken as correct. Want of agreement 

 between the two values would indicate that the liquid contained air or some 

 other impurity, assuming that the experiment had otherwise been carried out 

 satisfaotoril3^ 



' Trans. Chem. Soc, Ixiii. p. 1199, 1893. =Ann. de Toulouse, 1891. 



^Compt. rend., cxiv., p. 1093, 1892. 



