and the Critical Volumes of Liquids. 213 



oxide makes the body more transparent, and thereby favours 

 the observations. 



It is evident that this apparatus enables the observer to 

 determine the density of the fluid gases condensed in the 

 tube F ; since he has only to measure the volume of fluid 

 in F, and to collect, by means of the stopcock B, the whole 

 volume of gas given by the fluid and condensed vapour, which 

 gives an accurate determination of the total weight of substance 

 distributed bet,ween fluid and vapour in the whole apparatus. 

 The amount of substance which is required to produce the 

 vapour is easily found by observing the vapour-pressure of the 

 liquid gas before expanding it into gas for the volume mea- 

 surement ; and while keeping shut the stopcock B, by opening 

 A suddenly until this pressure is just reached -and then instantly 

 shutting off the receiver. If this volume of gas is now mea- 

 sured by opening B as before, the difference between the two 

 volumes thus collected will correspond to the real weight of 

 substance in the liquid state. A rough experiment with 

 oxygen near the critical point gave the density 0"65. 



As to the most convenient substance for use as a cooling 

 agent, I am still of opinion that marsh-gas would be the best ; 

 and I may take the opportunity of pointing out that the em- 

 ployment of this body was suggested by me in a communi- 

 cation made to the Chemical Section of the British Association 

 in 1883. The following extract from ' Nature/ of October 4, 

 1883, will prove that my experiments with liquid marsh-gas 

 were made a year in advance of those made recently by M. 

 Cailletet * and M. Wroblewskif:— 



" Professor Dewar pointed out an important relation 

 between the critical temperatures and pressures of volatile 

 liquids and their molecular volumes. The ratio of the critical 

 temperature to the critical pressure is proportional to the 

 molecular volume, so that the determination of the critical 

 temperature and pressure of a substance gives us a perfectly 

 independent measure of the molecular volumes. Prof. Dewar 

 pointed out the great advantage of employing a liquid of low 

 critical temperature and pressure such as liquid marsh- gas for 

 producing exceedingly low temperature. He hoped to be able 

 to approach the absolute zero by the evaporation of liquefied 

 marsh-gas whose critical temperature was less than —100° C, 

 and whose critical pressure was only 39 atmospheres." 



I ought to mention that the marsh-gas used in my experi- 



* u Sur femploi du Formene pour la production des tres basses tem- 

 peratures," Comptes Rendus, June 30, 1884. 



t " Sur les proprietes du gaz des marais liquide, et sur son eniploi 

 comme refrigerant," Comptes JZendus, July 21, 1884. 



