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PROCEEDINGS OF THE AMERICAN ACADEMY. 



At the present time this is not a good way to get information about 

 C p for two reasons. In the first place, all of the most reliable set of 

 volume measurements yet made (Knoblauch, Linde and Klebe) lie 

 close to the saturation line, not one of them reaching either 50° of 

 superheat or 190° of temperature. No characteristic equation based 

 on them can be depended on at points far out in the superheated 

 region. And in the second place, Clausius' equation involves a second 

 derivative of the observed quantity v, and even the first derivative of 

 an empirically determined function is liable to relative errors much 



Figure 9. Knoblauch and Jakob's measurements of Cp, reduced to a 

 pressure of 1 kg. per sq. cm. by means of Clausius' equation, using the char- 

 acteristic equation developed by Linde to represent the volume measurements 

 of Knoblauch, Linde, and Klebe. The smallest circles correspond to the 

 highest original pressure (8 kg.), the next smallest to 6 kg., and so on. The 

 progressive departure from a single curve with increasing pressure is 

 marked. 



larger than any in the observed quantity itself, while a second deriva- 

 tive is still more uncertain. This is illustrated by the fact that a 

 characteristic equation of Tumlirz's form, which was shown by Linde 

 to represent Knoblauch's volume measurements within four fifths of 

 one per cent throughout their range, leads through Clausius' equation 

 to the startling result that C P does not vary at all with pressure at 

 constant temperature, whereas it is known to vary within that same 

 range by something like 60 per cent of its initial value. 



The contention that even the best possible representation of Knob- 

 lauch's volume measurements is still too inaccurate to give reliable 

 values of Cp, through Clausius' equation, can be further substantiated 

 by an examination of the experimental data already described. Knob- 

 lauch and Jakob made observations on Cp at four pressures, all greater 



