724: Prof. A. W. Porter on tlie Isopiestic Temperatures 



being used as ordinates and the corresponding inversion tem- 

 peratures U as abscissae. The curve thus represents the 

 relation connecting the inversion temperature of the gas 

 under consideration with its initial pressure. The curves 

 fully confirm the conclusion of Witkowski and Porter that 

 the inversion temperature is a function of the pressure. The 

 values of the inversion temperature for air calculated by 

 Witkowski by the aid of the empirical formula given by Rose- 

 Innes ( + 360°), and by means of that of van der Waals 

 ( + 500°) are, it is true, somewhat high when compared with 

 those obtained by myself : but it must be remembered that 

 the second value ( + 500°) was calculated on the assumption 

 of a small pressure-difference (1 atmo.), whereas my figures 

 refer to the integral value of the Joule-Kelvin effect when a 

 gas expands from a high pressure to 1 atmo. 



In conclusion, I would refer to the connexion which 

 appears to exist between the shape of the curve for air and 

 the behaviour of this gas in a liquefying apparatus. By 

 means of the apparatus described by me in 1902 *, which I 

 am in the habit of using at lectures for the demonstration of 

 the liquefaction of air, it is easy to show that liquefaction 

 takes place only so long as the initial pressure does not fall 

 below 80 atmos. ; a further expansion from pressures which 

 are below 80 atmos. is entirely without result. From the 

 accompanying figure it is seen that precisely at the point 

 corresponding to a pressure of 80 atmos. the curve takes a 

 sharp bend, and that at this point a sudden drop takes place 

 in the temperature of inversion, whereby also the cooling- 

 effect rapidly decreases, the behaviour of air beyond this 

 point approaching in this respect more and more nearly that 

 of hydrogen, whose inversion temperature is very low. 



Chemical Institute of the Jagellon University, Cracow. 



LXV. On the Isopiestic Temperatures of Saturated Vapours 

 of Different Substances. By Alfred W. Porter, B.Sc, 

 Fellow of, and Assistant Professor of Physics in, University 

 College, London f. 



IN the Philosophical Magazine for Jan. 1886 Ramsay and 

 • Young published a law (known by their names) con- 

 necting the temperatures at which different saturated vapours 

 have the same vapour-pressure. The law has been employed 

 in two somewhat different forms. One only of these forms 



* K. Olszewski, Bull Acad. Crac. 1902, p. 623. 

 t Communicated by the Author. 



