The Liquefaction of Gases. 23 



conversely that increase of pressure would tend to form liquid 

 from gas. We also note incidentally that evaporation is here 

 also attended by loss of heat, producing in this case actual 

 freezing of the liquid. Thus we see that the two conditions 

 tending towards liquefaction of gases are pressure and cold. 



Before Andrews's time it was tacitly assumed that any defect 

 in one of these could be made up for by increasing the other. 

 If too little cold, more pressure would cause liquefaction. 

 Andrews, however, discovered that for each gas there was a 

 certain temperature above which no amount of pressure would 

 liquefy that gas. This temperature he called the critical 

 temperature for that particular gas. Below that temperature 

 and at a sufficient pressure, called the critical pressure, the gas 

 would liquefy with a decrease of volume. Above it no 

 liquefaction could be observed ; yet when not much above it 

 there was as the pressure increased a somewhat more rapid 

 decrease in volume, than would correspond with the behaviour 

 of what is called a perfect gas. Yet, again, when above the 

 critical point, and therefore in a gaseous state, the gas, when 

 reduced to about the volume which it would have occupied 

 when liquefied at a lower temperature, yielded only slightly to 

 further pressure. As regards its elasticity, it behaved then as 

 a liquid. These researches were carried out with carbonic acid 

 as an experimental agent, and in them is illustrated what 

 Andrews aptly called the continuity of the gaseous and liquid 

 states of matter. As he says, " From carbonic acid as a perfect 

 gas to carbonic acid as a perfect liquid the transition may be 

 accomplished by a continuous process." 



Andrews, who began with an attempt, unsuccessful as it was> 

 to merely liquefy the most refractory gases — oxygen, hydrogen, 

 nitrogen, etc. — was thus led aside to a path rich in scientific 

 interest — perhaps richer even than that which he set out to 

 pursue. This was left to later investigators. 



In 1877 Pictet achieved the liquefaction of oxygen by 

 combined pressure and cold, produced by elaborate and costly 

 machinery. A few days only after Pictet's success Cailletet 



