DAVIS. — CERTAIN THERMAL PROPERTIES OF STEAM. 303 



6. The reconciliation, through Clausius' thermodynamic relation, of 

 the accepted volume and specific heat measurements in the superheated 

 region is impossible. This is probably the most important of the out- 

 standing problems in this field. 



All these conclusions have been embodied in the C p diagram which is 

 the basis of the Steam Tables already mentioned, 49 and it is partly for 

 the purpose of gathering in one place all of the underlying evidence 

 that justifies those tables, much of it unsuitable for presentation there, 

 that this section of the present paper has been written. The C p curves 

 which were used were as faithful a translation and extrapolation of 

 Knoblauch's curves as possible, except for the differences stated above. 

 In particular they reproduced the tremendous rise of his saturation 

 curve at even moderately high pressures and temperatures. It is 

 probable that this feature of Knoblauch's curves, although near enough 

 to the truth to satisfy the present needs of engineering practise, will 

 have to be revised later. It is, however, the only rational guess yet 

 published, and it is not worth while to cumber the literature with any 

 more " harmonized " sets of C p values at high pressures until there is 

 something definite to build on. The problem of determining the true 

 course of the high pressure end of the saturation curve on the C p dia- 

 gram is second in importance only to that mentioned at the end of 

 the summary just above. 



5. Clausius' "Specific Heat of Saturated Steam." 



This section will be devoted to a revision of a computation first 

 made by Clausius, which, although no longer of especial importance, 

 is usually of considerable interest to students of thermodynamics. In 

 the sixth chapter of the first volume of his " Mechanical Theory of 

 Heat " he defines the specific heat of saturated steam as the quantity 

 of heat that must be added to saturated steam at any temperature to 

 turn it into saturated steam one degree hotter, account being taken of 

 the fact that it will have to be compressed to keep it saturated. For 

 steam and for most other substances it is negative at ordinary tempera- 

 tures, because the work of compression is more than enough to provide 

 the corresponding increase in the internal energy. But in the case of 

 most substances including steam it is at ordinary temperatures an 

 increasing function of the temperature and may, therefore, pass 

 through zero and become positive if tltfe temperature is sufficiently 

 raised. This Clausius found to be actually the case for ether at 

 ordinary temperatures and for chloroform above 130°, and the ex- 



49 See page 97 of the Tables. 



