Ixiv INTRODUCTION. 



versity of Leiden. M. Thiesen,^ making use of the data of Scheel and Heuse, 

 has developed a formula for vapor pressures over ice. This is given by the 

 equation, 



(2) logio^ = logio ^0 + 9-632(1-0.000350 y 

 where 



^0 = 4-5785 and T=2j2, + t, 

 the vapor pressures, e, being in millimeters and temperatures, t, in degrees 

 Centigrade. 



For convenience in computing this equation, for metric units it may be 

 written 



/g.632 — 0.00^3712 A 



(3) log.o g^ 0.660-2+ ( ^ 273 + / )*• 



For English units the equation becomes 



000 / 9.69 1 93 — 0.00 1 87289 /i\ , 



(4) log.. ..= 1.255888+ { ^ %S3.^ + ,/ V ('.-32) 



^= vapor pressure in millimeters. 

 ex = vapor pressure in inches. 

 ^ = degrees Centigrade. 

 ti = degrees Fahrenheit. 



Although the Scheel and Heuse observations extended down to —67^9 C, 

 the pressure readings between —60° C. and that temperature were not very 

 accurate, being discarded by Thiesen ^ in obtaining the constants in equation 

 (2). 



Nernst has made determinations of vapor pressure down to at least —50° C, 

 good agreement being found with Scheel and Heuse's measurements. By 

 making use of accurate determinations of the heat of vaporization of ice at 

 0° C, and attributing the deviations of water vapor from the gas laws to the 

 existence of double water molecules - Nernst with the collaboration of H. Levy 

 has found for the vapor pressure over ice the formula 



(5) logio e=- ^^"'^ + 1.75 logio r- 0.002 10 r + 6.5343, 



where ^= vapor pressure in mm. of mercury 



and 7=273.09 + ^ 



t = degrees Centigrade. 

 This formula has been checked by the accurate determinations of Weber 

 the results of whose observations show good agreement with the values 



1 Thiesen, M. Die Dampfspannung iiber Eis. (Mitteilung aus der Physikalisch — 

 Technischen Reichsanstalt.) Annalen der Physik, vol. 29, p. 1057, 1909. 

 - Weber, S. Loc. cit., pp. 50-52. 

 Knudson, M. Annalen der Physik. Vierte Folge, Band 44, p. 536, 1914. 



