CH. xxxvn. LIQUEFACTION OF GASES. 407 



pint, and yet it will remain unliquefied. But directly it is 

 reduced below 30-92 Cent., which is its critical point, the 

 molecules do not move with sufficient energy to resist 

 cohesion, and the vapour becomes liquid at once without 

 suddenly changing its volume or giving out any heat. 



Now, those gases, oxygen, hydrogen, and nitrogen, which 

 were called ' permanent ' gases, have each their ' critical 

 point,' so enormously below all ordinary temperatures that 

 no one until December 1877 had succeeded in obtaining 

 cold and pressure enough to reduce them to liquids. But 

 in that month M. Raoul Pictet, of Geneva, and M. Cail- 

 letet, an iron-founder at Chatillon sur Seine, were fortunate 

 enough to liquefy oxygen, each of them independently of 

 the other. 



M. Cailletet put his oxygen in a small tube under 

 enormous pressure, surrounded it by a freezing mixture, 

 and then let it suddenly escape from the tube. The gas, 

 already intensely cold (29 below o Cent), became so much 

 colder by expanding as it rushed out, that it liquefied into 

 minute drops, giving the appearance of a mist. M. Pictet 

 had a more elaborate apparatus, in which, by a double pro- 

 cess of freezing, by means of sulphurous acid and carbonic 

 acid, he obtained a cold of 140 Cent, below the freezing 

 point, while he exerted a pressure upon the gas 650 times 

 greater than the pressure of our atmosphere. He then 

 opened the stopcock, and the oxygen, set free, shot out in 

 a liquid stream. 



The next step was made by M. Cailletet on the last day 

 of 1877, when he compressed not only nitrogen and hydro- 

 gen, but also atmospheric air into liquids, before the leading 

 scientific men of Paris. The nitrogen appeared in drops, 

 but the hydrogen only as a faint mist. Lastly, M. Pictet, 

 on January n, 1878, under a pressure 650 times that oi 



