Evaporation and Dissociation, 437 



the ordinates and abscissae being respectively pressures and 

 volumes. It was possible to read pressure accurately to 

 within 20 millim. ; and volume, up to a volume of 3*1, to 

 within O'OOl cubic centim. per gram ; and, at volumes greater 

 than 3*1 cubic centim. per gram, to 0*01 cubic centim. per 

 gram. Pressures corresponding to each isothermal were then 

 read off on the equal volume-lines, from curves constructed to 

 fit the experimental points as accurately as could be drawn 

 with the help of engineers' curves. These pressures and 

 temperatures were then mapped as ordinates and abscissae ; 

 and it was found that points corresponding to each volume 

 lay in a straight line. Again, two points were chosen on 

 these equal volume-lines, as far apart as the scale of the dia- 

 gram would permit, and the values of the change of pressure 



per unit change of temperature, -~ } were ascertained for each 



separate volume chosen. To eliminate irregularities, these 

 values were smoothed graphically ; but it was difficult to find 

 any very satisfactory method. The method employed for 

 ether, which we found to give the best results, was to map as 

 ordinates the ratios between these values, and similar values 

 calculated on the supposition that the gas or liquid followed 

 the usual gaseous laws, against the reciprocals of the volumes 

 as abscissae. A curve was then drawn, taking a mean course 



among the actual points, and the values of -~- were calculated 



Clt 7 



from readings at definite volumes. This expression, — , is the 

 G r ' dt' 



b of our formula. Having thus obtained the most probable 

 value of b for each volume, the value of a at each volume was 

 ascertained by calculation from each individual point read 

 from the original curves, and at each volume the mean of all 

 was chosen. 



Isothermals were then calculated by means of the equation 

 p = bT — a, T being kept constant ; and those values of a and 

 b corresponding to the volumes required being selected. These 

 calculated isothermals are shown on Plate VII. ; and the lines 

 of equal volume, or isochors*, on Plate VIII. It is evident, 

 from inspection of the former, that the calculated lines corre- 

 spond as closely as possible with the actual observations. 

 ^ It is necessary now to give the data on which these deduc- 

 tions are based. The following Table gives those points 

 corresponding to lines of equal volume read from the diagram 

 constructed from experimental observations. 



* From ta-os, equal, and xo>peTz/ 7 to contain. Another suitable word 

 would he " isoplether but we have Professor Jowett's preference for the 

 one selected. Either of these terms seems preferable to that (isometric*) 

 already proposed. 



