102 PROF. c. n.i:\\i N U:XKI.\ AND MR. P. R. PYE ON THE 



To construct the constant-pressure lines in the superheated area he assumed (2) 

 that the specific heat at constant volume, C w was constant, which he points out is 

 not ,,.iit,- tin,-. ( )n this assumption the constant- volume lines on the 6<f> diagram are 

 the same as those for a perfect gas (logarithmic curves). The constant-pressure 

 curves were then determined from these constant-volume curves by the p.v. 6 curves 

 observed by AMAUAT. MOLLIER does not draw I lines on his Q$ diagram. His 

 values of I are given in his 10 diagram (3), which is outside the range of the present 



discussion. 



The agreement between the two diagrams, constructed by such widely different 

 methods, is remarkable, the more so because MOLLIEH had no data to go on below 

 0" C., except REONAULT'S very imperfect pressure-temperature curve. The following 

 brief comparison between the two methods shows how widely they differ. 



Our diagram is based in the most direct manner possible on experiments at the 

 temperatures and pressures represented, whereas MOLLIER'S is based on a mathe- 

 matical equation obtained from experiments at higher temperatures. Our diagram 

 is constructed from the left, his from the right hand. Our diagram is based on 

 measurements of heat, his on measurements of density and pressure. 



The two diagrams are superposed in fig. 13, and values of and L are given in 

 Table XVII. for comparison. In Table XVIII. our data are arranged for direct 

 comparison with the tables given by MOLLIER (l) and EWING (4). 



The differences between the diagrams are due to the differences in the various data, 

 which have been already discussed. The authors take this opportunity of expressing 

 their great admiration for the judgment and skill by which Dr. MOLLIER has selected 

 the most reliable data and devised mathematical methods capable of giving results 

 which direct experiments have confirmed so closely. 



SUMMARY. 



The authors have reconstructed the d<j> diagram by a new method and, at the same 

 time, extended it from -30 C. to -50 C. The reconstruction is based on direct 

 heat measurements, and the results are believed to be more accurate than those 

 arrived at indirectly by MOLLIER. 



The observations include the direct measurement of the following quantities : 



The latent heat ; 



The total heat of the liquid ; 



The specific heat of the gas ; 



The dilatation and elasticity of the liquid ; 



The Joule-Thomson effect for the liquid. 



From these direct measurements the following quantities have been calculated : 

 The specific volume of the saturated vapour ; 

 The specific heat of the liquid at constant pressure. 



