EXPEEIMENTAL KNOWLEDGE OF THE PEOPERTIES OF MATTER. 131 



liquefied portion of a gas may be readily determined, and in particular 



T 

 the density at the critical temperature and pressure. The values of :g are 



proportional to the molecular volumes of the gases at the critical point. 



We have thus the means of determining the critical temperature, 

 pressure, and volume of a gas or liquid. These are the most important 

 data for each substance, and the most important points of reference when 

 we compare different substances, which are gasifiable, with one another. 



It has already been mentioned, p. 129, that Sarrau deduced the critical 

 temperatures and pressures of oxygen and nitrogen by applying Clausius' 

 formula to Amagat's results. 



Sarrau found for oxygen ^^= — 105'4° ; ^(.=48"7 atmos ; • 

 and for nitrogen i^,= — 12.3'8°; ^^=42-1 atmos. 



The values found by Wroblewski and Oblewski for oxygen arc 

 respectively : — ^ 



Oxygen, Wroblewski, Z^^ — 113°; j)^=50 atmos. 



Oblewski, ;;,= -118-8°; j?,=50-8 atmos. \ 



The pressure found by both observers does not differ much from that cal- 

 culated by Sarrau ; but the temperature calculated is considerably higher 

 than that observed by either. 



Hydrogen has, so far as I know, not been examined as yet in the liquid 

 state ; but if not, there can be little doubt that it soon will be. Wroblew- 

 ski, by means of nitrogen boiling in a vacuum, cooled hydrogen to a tem- 

 perature 208° — 211°, at a pressure 180 — 190 atmos, and found on 

 suddenly releasing the pressure a grey mist form, which is due no doubt 

 to the formation of liquid hydrogen in a very fine state of division. 



Sarrau deduced from Clausius' formula for hydrogen the critical tem- 

 perature — 174°, and critical pressure 98-9 atmospheres. The ratio of 

 absolute critical temperature to critical pressure is therefore about I'O. 



Again Olszewski - has obtained his lowest temperatures by the eva- 

 poration of solid nitrogen under a pressure of 4 mm., a temperature 

 — 225° having been thus registered by his hydrogen thermometer, which 

 perhaps cannot give accurate temperatures in these extreme circumstances. 

 However, hydrogen does not seem to liquefy at this temperature ; at least 

 no meniscus was seen at —220° at pressures up to 180 atmos; still the 

 hydrogen- thermometer might, and probably would, register too low. 



The solid nitrogen which Olszewski used was obtained by evaporation of 

 the liquid nitrogen at 4 mm. in a glass tube surrounded by liquid oxygen. 



The temperature at which nitrogen solidifies is, according' to 

 Wroblewski,3 -203°. 



Dewar has recently obtained solid oxygen, but details have not yet 

 been published. 



Pawlewski had stated, as an empirical law, that the difference between 

 the critical temperature and the boiling-point is constant. This has been 

 found to be by no means true. Vincent and Chappuis ^ find the critical 

 temperature, boiling-point, critical pressure, and the ratio T/P, where T 

 is the absolute critical temperature, for hydric chloride, methyl chloride, 

 ethyl chloride, ammonia, and methyl-, dimethyl-, and trimethyl-amines ; 

 the differences between the centigrade critical temperatures and boilino-. 

 points for these in order are 86-5°, 165-2°, 195°, 169-5°, 157°, 155°, and 

 ' C.B. xcvii. 309 ; c. 350. ^ jj,i^ pji 



2 Ihid. ci. p. 238. * Ibid. ci. 427. 



k2 



