£4 Thermodynamical Correction oj the Gas-Thermometer. 



point. But as the effect amounts to little more than a 

 hundredth of a degree per atmo it would be easily masked 

 by any slight impurity in the hydrogen, so that little stress 

 •can be laid on this observation. Assuming- that the heating- 

 effect Q is constant up to a pressure of 117 atmos, the 

 •observations of Olzewski would require c = 2*0 c.c, b = 8'5 c.c. 

 if we take w==l"5, and c — h=— 6"5 c.c. These values 

 would make the heating-effect at 0° C. Q= — *024° per atmo, 

 which is rather smaller than that observed by Joule and 

 Thomson, but the difference is hardly beyond the possible limits 

 ■of error. The absolute zero-correction for the constant-volume 

 thermometer would be larger in the proportion of 2 to 1*5, 

 but that of the constant-pressure thermometer would be 

 smaller, agreeing slightly better with experiment. The value 

 of c for hydrogen probably lies between 1 and 2 c. c, but we 

 €an hardly expect to be able to determine it more closely with 

 certainty, since it is of the order of one part in 10,000 only 

 of the specific volume at 0° C. and 760 mm. pressure. 



21. Summary of Conclusions. 



(1) The deviations of a gas or vapour from the ideal state 

 ■at moderate pressures can be represented by an equation of 

 the type v — b=B.0/p — c, in which the " covolume " b is 

 constant, and the " co-aggregation- volume " c is a function 

 of the temperature only. This conclusion follows from the 

 observed form of the isothermals combined with the observa- 

 tion that the " cooling- effect ■" is independent of the pressure; 

 but it could not be deduced from either observation separately. 



(2) The value of the Absolute Zero may be approximately 

 deduced from a knowledge of the cooling-effect Q and the 

 specific heat S at or near 50° C. without any knowledge of 

 the mode of variation of S and Q with temperature. But 

 the determination of the scale-correction of the gas-ther- 

 mometer essentially requires a knowledge of the mode of 

 variation with temperature. 



(3) The simplest assumption with regard to the mode of 

 variation of c with temperature is that it varies inversely as 

 the nth power of 0, or that c = c (0 /6) n . The value of n is 

 different for different types of co-aggregation or for different 

 kinds of molecules. The law of Corresponding States must 

 be restricted to molecules of the same type which coaggregate 

 in a similar manner. 



(4) The index n may be interpreted as half the number 

 of degrees of freedom lost by a molecule in coaggregation, 

 the energy of flight of a molecule representing three degrees 

 of freedom. 



