HYGROMETRICAL TABLES. Ixiii 



because the same result is attained by simply passing the curve through a 

 point more accurately known and as near as may be to the critical point. 

 This is equivalent to defining tt and by an " equation of condition." 

 Another " equation of condition " fixes the pressure at the boiling point which 

 by definition must be 760 mm. From the considerations given on page xv 

 computations are greatly facilitated by taking all temperatures on the approxi- 

 mate absolute scale represented by 7 = 273 X^°. 



A careful preliminary analysis of the observational data in the vicin- 

 ity of the critical temperature resulted in assigning values to 6 and tt as 

 follows : 



^ = 643°, log. 77=5.1959000 



It is emphasized here again that these data do not represent critical tem- 

 perature conditions, but simply a convenient point on the pressure curve 

 slightly below the critical temperature, the value of which is fixed with 

 considerable accuracy by the observational data. 



The value of the constant A was fixed by the equation of condition, 

 ^ = 760 mm. when 7^ = 373 (X=— 8). The remaining constants (b, m, n, s) 

 are computed by the method of least squares. The results are as follows: 



^ = 3-1473172 



& = . 00295944 

 7n=. 0004 1 91 398 



n = .0000001829924 



s— .00000008243516 



The number of significant figures in the constants is obviously greater 

 than the accuracy of the data justifies, but is justified to facilitate compu- 

 tation and to secure accuracy in the interpolation of values which should 

 themselves be as accurate as the data. 



Observations of the pressure of aqueous vapor over ice have not been as 

 numerous as those over water. Among the observations which have been 

 used in recent times for the development of formulas to express the values 

 of vapor pressures over ice there may be mentioned those of K. Scheel and 

 W. Heuse ^ at the Physikalisch-Technischen Reichsanstalt at Charlottenburg, 

 those of W. Nernst ^ at the Physikalisch-Chemischen Institut of the University 

 of Berlin, and those of S. Weber ^ at the Physical Laboratory of the Uni- 



1 Scheel, K., and Heuse, W., op. cit., p. Ixi. 



2 Nernst, W. Verhandlungen der Deutschen Physikalischen Gesellshaft, vol. 11, 

 no. 15, p. 313, Aug. 15, 1909. 



Nernst, W. Kinetische Theorie fester Korper ; Vortrage iiber die kinetische Theorie 

 der Materie und der Elektrizitat. B. G. Teubner. 



3 Weber, S. Communications from the Physical Laboratory at the University of 

 Leiden, no. 150; p. 37, 



