128 Henry Riddell on 



about 30°C, there is in each case a point at which the volume 

 lessens as the screws are forced into the liquid in the reservoir 

 tubes, while the pressure does not increase until a certain volume 

 is reached at which this action ceases, and the pressure abruptly 

 starts again to rise. The first point marks the beginning of the 

 change into the liquid condition, while the second marks the 

 completion of the change, when the gas is all liquefied, and the 

 increasing pressure is only accompanied by such reduction of 

 volume as corresponds with the gradual compression of the liquid 

 thus formed. If the CO2 were quite pure the line showing the 

 process of liquefaction would be truly horizontal, but it proved 

 to be impossible to remove the last vestige of air from the tubes, 

 and it was found that from one five-hundredth to one thousandth 

 part of the volume consisted of air, which is not liquefied under 

 the conditions of the experiment, so that the line has a slight 

 upward tendency corresponding to the increased pressure required 

 to reduce the volnme of the air itself. 



It will be noticed that, as' the temperature is greater, this 

 horizontal line in the isothermal curve becomes shorter, so that 

 at a temperatui-e approximative to 30°C., it becomes inlinitely 

 short : and at this point the curve has a point of inflexion. This 

 is a critical point of liquefaction, and the 5)articular temperature 

 belonging to this isothermal line is called the critical tempera- 

 ture. It was defined by Andrews as that temperature below 

 which liquefaction visibly occurs, and above which no pressure, 

 however great, will cause a partial liquefaction, or at which the 

 liquid can exist in presence of the gas He pointed out that the 

 passage from the gaseous to the liquid condition may be made 

 absolutely continuous, and for this reason chose as a title for his 

 statement of results " The Continuity of the Liquid and Gaseous 

 States." 



The critical temperature is often defined as that tem- 

 perature above which no pressure, however great, will cause 

 liquefaction to take place." 



