LECTURE XV.] HISTORY OF CHEMISTRY. 32 1 



Eight years later, the celebrated paper of Andrews 142 ap- 

 peared, in which the connection between pressure, volume, and 

 temperature was accurately examined in the case of carbonic 

 anhydride, which, it was shown, could not be liquefied at tem- 

 peratures above 30.92 C. Andrews called this the critical 

 temperature ; and, further, he designated as critical pressure 

 the pressure which is just sufficient to bring about liquefaction 

 at a temperature infinitesimally below the critical temperature. 

 Andrews's observations enabled him to draw isotherms for car- 

 bonic anhydride at different temperatures, which exhibited the 

 relations between pressure and volume. When this had been 

 done, it appeared that the curves were discontinuous below 

 30.92, and consisted of different parts. Although slight 

 changes of curvature are observed in the isotherms for tem- 

 peratures just immediately above 30.92, these are no longer 

 observed at 48, the curve at that temperature approximately 

 corresponding, throughout its entire length, to the equation 

 which holds for gases 



PC -a 



that is, it approximates to a rectangular hyperbola. 



As a result of this investigation the definitions of vapour 

 and of permanent gas which had previously been adopted were 

 abandoned, and the word gas is now applied to every substance 

 in the gaseous state when heated above its critical temperature. 143 

 The continuity of the liquid and gaseous states is observable 

 when a liquid is heated under a pressure greater than the 

 critical pressure. In such a case a separation into liquid and 

 gas never takes place, but the liquid is transformed into gas 

 without the change giving rise to any noticeable heterogeneity. 144 



These investigations exercised a decided influence upon the 

 experiments on the condensation of gases. Faraday, as is well 

 known, was the first to turn his attention to this matter with 



142 Phil. Trans. 1869, 575. 143 Wroblewsky (Monatshefte. 7, 383) 

 afterwards attacked the conception of critical temperature ; hut the matter 

 cannot be further discussed here. 144 Compare Ostvvald, Allgemejne 

 Chemie. I, 267. 



X 



