8 ATTAINMENT OF VERY LOW TEMPERATURES. 



liquid and gaseous phases can connect. The adiabatic a'" b'" passes 

 through the critical point, so that in this case the liquid will first appear 

 at the critical pressure. For higher initial compressions the adiabatics 

 will lie on the left hand side of the critical point on the diagram, and 



Fig. I. — Andrews diagram. 



liquid will first appear under conditions represented by the point at 

 which they cut the horizontal line drawn through the critical point. 

 For high pressures the adiabatics will, of course, lie very close to- 

 gether, and the horizontal line will differ but slightly from the curve 

 joining the ends of the horizontal section of the isothermal. 



IV. The Application of the Joule-Thomson Effect to the 



Liquefaction of Air. 

 The change in temperature which takes place when a gas is allowed 

 to expand without performing effective external work was first studied 

 by Joule and Lord Kelvin in 1848. The gas compressed to about two 



