479 



My thanks are due fo Mr. G. HorsT, assistant at tlie Pliysioal 

 Laboratory, for making these tenipei-atnre determinations, and t'oi- 

 the calcuhxtions of the temperatnres. 



The determination of the tri|)ie point was made difficult by the 

 undercooling phenomena which constantly aj)peared. If, while the tem- 

 perature was just above the triple point, the temperature was slowly 

 allowed to fall, it repeatedly ha]>pened that the whole mass of liquid 

 argon crystallized out. By allowing the temperature to rise and fall 

 with extreme slowness, and waiting j)aiiently for the establishment 

 of the equilibrium. I at last succeeded in establishing a temperature 

 at which the three phases remained in e({uiiibrium with each other 

 for nearly two hours. During this period, the triple point was 

 determined with an admirably constant pressure and temperature. 

 This also gave me an opportunity of observing that the piece ot 

 solid argon sank in the liquid, and remained lying at the bottom 

 of the tube, so that in the triple point the density of solid argon is 

 greater than that of liquid argon. We shall presently be able to 

 make use of this observation, although I had no opportunity to 

 make a quantitative determination of the density of solid argon. 

 Concerning the calculation of the observation there is nothing partic- 

 ular to remark. An atmosphere in Leiden is reckoned at 75.9488 cm. 

 mercury. 



2. Representation of the observations by formulae. 

 In order to get a useful representation of the observations by 

 a formula, I first made use of Rankine and Bose's formula 



for the field of liquid-vapour as well as for that of solid-vapour. 

 The result of the testing of the values of the pressure calculated 

 from these formulae by the values observed, are found in table IL 



As can be seen in the table, the correspondence is fairly satis- 

 factory, especially in the part liquid-vapour, while it might be further 

 improved for instance by the application of tiie method of least 

 squares. 



In the second place I have tried to apply to my observations 

 Nernst's^) vapour pressure formula. We may regard it as a purely 

 empirical formula with 4 coefficients, written in the form 



A 



logp ^ ^^ BT ^ D loii T -^ C 



and then calculate the coefticients which correspond best to tlie 

 observations, without troubling about the theoretical significance of 



1) VV. Nernst, Göttinger Nachr. 1900, p. 1. 



