Dr. Thomas Andrews ; The Great Chemist 129 



It does not seem that this is a logical statement, nor did 

 Dr. Andrews ever teach in this form. It does not seem possible 

 to show that liquefaction has not taken place when the gas has 

 been sufficiently compressed above the critical temperature. It 

 seems better to define the critical temjieratuie as that above 

 which any conversion of gas to li([uid takes place with perfect 

 continuity. 



We may, however, frame a definition for " point of lique- 

 faction " by referring to the isothermal curves. In every 

 isothei'mal below the critical temjjerature there is a jjoint of the 

 curve in wliicli tlie diagi'am shows a break in the nature of the 

 line. Tliis corresponds to the point at which b'(iuefaction is 

 taking place, at which the pressure remains constant while the 

 volume decreases rapidly. If we accept the theoretical curves due 

 to James Thomson — explained before our Society fifty years 

 ago, and shown in figure II — there are two points on each of these 

 curves at which the tangents are horizontal. These are points of 

 liquefaction or the converse action. Between these points the 

 substance is unstable. It may exist in either state, but tends 

 very strongly to the condition usually apparent at the corres- 

 ponding pressure. On the isothermal belonging to the critical 

 temperature there is a point at which the two tangents coincide, 

 and this becomes a point of inflexion and the critical points, the 

 tangent becoming horizontal and also catting the curve. 



To put the result into mathematical language any tangent to 



the curve at which — = 0, marks a point of liquefaction, while 



d?p 

 at a critical point we have also -,—0 = 0. 



The isothermal curve is therefore one of the third degree, 

 and many equations have been framed to express this, some of 

 Avhich will be referred to later. 



These experiments and conclusions of Professor Andrews 

 form a great part of the foundation of the art of refrigeration, 



