186 Mr. W. Sutherland on the 



following equation : — 



pv = l ae» IT ; 



where ^ v 



a = 422«14, 6 = -012831, c = '83497, Z=376'5 ; 



the unit of pressure being that of a kilogramme weight per 

 square metre, and the unit of volume a cubic metre, v being 

 the volume of a kilogramme of hydrogen. Amagat's unit of 

 volume is 3^2 of that occupied by the gas at 0° and 760 

 millim. The critical temperature given by this equation is 

 T = 30°'56 or t= -242°'4 ; the critical pressure is 22 atmo- 

 spheres, and the critical volume of a kilogramme of hydrogen 

 is *02076 cubic metre. 



Olszewski (Comptes Rendus, 101. p. 238) states that he has 

 submitted hydrogen to pressures varying from 20 to 180 

 atmospheres at —220° C. without getting it liquefied. The 

 temperature — 220 c was measured by means of a hydrogen- 

 thermometer, on the assumption, I presume, that hydrogen 

 under pressures near that of 1 atmosphere obeys Charles's or 

 Gay-Lussac's law right down to that low temperature. The 

 above equation fully justifies Olszewski's assumption, and his 

 reliance on the hydrogen-thermometer ; for whether it is used 

 as a constant-volume or constant-pressure thermometer, its 

 indications for volume or pressure near the normal, as calcu- 

 lated by Charles's or Gay-Lussac's law, are not appreciably 

 different from those calculated from the above equation. This 

 is an important fact in Physics — that there should exist a 

 substance capable of giving regular thermometric measure- 

 ments almost down to absolute zero. 



We may regard the above-determined critical temperature 

 of hydrogen as remarkably near the truth, considering how 

 far beyond Amagat's experimental range we are extrapolating. 

 Wroblewski (Comptes Rendus, 100. p. 979), by suddenly ex- 

 panding hydrogen cooled down to the melting-point of solid 

 nitrogen, from a pressure of 180 atmospheres to that of 1 

 atmosphere, got signs of liquefaction such as Olszewski had 

 also obtained in a similar manner, while his thermopile in the 

 fluid indicated —208° or —211°; but he cannot say how 

 nearly the thermopile itself actually came to the true tempe- 

 rature of ebullition of hydrogen under a pressure of 1 atmo- 

 sphere. 



The cooling effect for hydrogen at 0° C, expanding through 

 plugs under a pressure-excess of 1 00 inches of mercury, is 

 found from the above equation to be *2°. Regnault found a 

 cooling of -04°, and Thomson and Joule (part ii.) found a 

 cooling of *07° ; but in their latest determination the effect 

 found for hydrogen was an irregular heating-effect. This 



