188 



HEAT. 



Going back to Fig. 108 it will be seen that the length of the horizontal 

 portion of the isothermal decreases as the temperature rises, so that we 

 should expect from this diagram alone that above some limiting isother- 

 mal the flat parts of the curve will cease to exist that is, that the two 

 states, liquid and vapour, will gradually approach each other, as the 

 temperature rises until all distinction ceases. 



Critical Point. This merging of the two states into one, above a 

 certain temperature, was first suggested by some experiments carried 

 out by Cagniard de la Tour in 1822. He showed that if a proper quantity 



of alcohol was heated 

 in a confined space, in 

 the presence of its own 

 vapour only, up to a 

 certain temperature the 

 contents of the vessel 

 were partly liquid, 

 partly gaseous, but that 

 at this temperature 

 about 225 C. the 

 surface of separation 

 between the two dis- 

 appeared, and above it 

 the contents of the 

 vessel were entirely 

 homogeneous, the two 

 states appearing to 

 meet. Before the dis- 

 appearance of the sur- 

 face of separation the 

 surface - tension gradu- 

 ally diminished, tend- 

 ing to show that the 

 distinction was disap- 

 pearing. He observed 

 the same phenomenon 

 with other liquids, and 





e\ 



1 



\ 



FIG. 



110. Continuous Form of Isothermal 

 suggested by James Thomson. 



Faraday continued the 

 researches, making the 



important suggestion that for such gases as oxygen, hydrogen, and 

 nitrogen, this limiting temperature is far below ordinary temperatures, 

 so that they cannot be liquefied by pressure alone (Researches in Chemistry 

 and Physics, p. 99.) 



But our definite knowledge on the subject was greatly increased by 

 the researches of Andrews, who made a careful map of the isothermals 

 of carbon dioxide from actual experiment, verifying the existence of the 

 limiting or critical temperature, and showing the shape of the isothermals 

 both above and below this critical temperature. 



His apparatus consisted of a long glass tube (Fig. Ill), about 2*5 mm. 

 in diameter from c to b, and with a capillary bore from b to a, the whole 

 being carefully calibrated. A current of dry carbonic acid gas was 

 passed through the tube for some hours, until the proportion of air in the 



