892 Prof. A. W. Porter on the Inversion- Points for 



These numbers must be regarded as approximate only; 

 partly on account of the uncertainty in the curves themselves 

 for each of which there are five points only; and partly owing 

 to the uncertainty of the critical data. The uncertainty in 

 regard to the critical data might be removed by employing 

 actual instead, of " reduced " values of p, v, and T, but 

 at present it is general relations only that are sought 

 for, and the results are accurate enough to give valuable 

 information. 



Plot the reduced pressures and temperatures of the in- 

 version-points against one another (fig. 3). Notice first that 

 there is an indication of a maximum pressure for which 

 inversion can occur. In fact the 400-atmosphere curve (fig. 2) 

 is not far from this maximum pressure. This is the expla- 

 nation of the wriggle in this particular curve. Although 

 it was perfectly easy to draw curves with curvature of the 



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same sign for the other pressures represented on fig. 2, it was 

 impossible to do so for the 400-atmosphere curve. The point 

 for 16° seemed to wander from a smooth curve drawn through 

 the remaining points as though there was greater error 

 attached to it. The recognition of the possibility of two 

 inversion-points for the same pressure cleared away the diffi- 

 culty that was felt at first, and revealed the faithfulness with 

 which Amagat's work had been carried out. The apparent 

 error of this point was precisely such as might easily have 

 been removed by the smoothing-out processes to which so 

 many observational results are subjected. On fig. 3 is also 

 plotted the curve corresponding to van der Waals' equation 

 of state. It will be seen that there is no quantitative agree- 

 ment between it and the curve derived from experiment. On 



