CHANGE OF STATE LIQUID VAPOUR. 



189 



gas issuing from the tube was small and constant. It was found always 

 to be not less than y^n^j- of the whole, and the proportion had to be 

 determined and allowed for. The end a was then sealed, the other end 

 closed and af terwards opened under mercury. By suitable manipulation, 



c b a. 



FIG. 111. Andrews' Tube for Experiments on Carbon Dioxide. 



partly by heating, partly by reduction of external pressure, about one- 

 fourth of the gas was expelled, and a short column of mercury intro- 

 duced into the tube cb to act as stopper and index. The tube was then 

 placed in position with the open end c in the compression chamber 

 (Fig. 112), which was filled with water. By means of the screw S, any 

 desired pressure could be applied, the amount being 

 measured by a second exactly similar tube and com- 

 pression-chamber, containing air instead of carbonic acid 

 gas, and placed by the side, a cross tube connecting the 

 two chambers. Each tube was surrounded by a water- 

 bath with plate glass sides, not shown in the figure, that 

 round the air tube being kept at a constant temperature, 

 while that round the carbonic acid gas was raised to any 

 desired temperature. 



The diagram (Fig. 113) taken from Maxwell's Theory 

 of Heat (5th ed., p. 120) shows Andrews' results. The 

 13'1 and 21'5 isothermals exhibit the normal change 

 from the all-gas curve on the right to a horizontal line, 

 indicating condensation at constant pressure and a 

 coexistence of the two states, followed by a steep rise of 

 the all-liquid curve. In the actual results, through a 

 small trace of air in the tube, the corners were rounded 

 off and the flat part of the isothermals sloped slightly 

 upwards, showing that the last part of the condensation 

 required a greater pressure than the first. The higher 

 isothermals slope upwards throughout, showing no con- 

 densation at all. During the compression the substance 

 remains homogeneous, and the only relic of the co- LgJ 



existence of two states is in the diminished slope at one 

 part of the isothermal. Even this has disappeared at 

 48*1. By several experiments the lowest temperature 

 at which condensation does not occur was fixed at 30'92. FIG. 112. Dia- 

 This therefore is the critical temperature for carbon grammatic Re- 

 dioxide. Anlrews^Com! 



If we draw a curve (dotted in Fig. 113), the " border pression Appa- 

 curve," through the points where the isothermals below ratus. 

 this temperature change into and from the horizontal 

 straight line, its area includes all the conditions of coexistence of gas and 

 liquid at one temperature. To the right the substance is a gas, but one 

 for which the volume decreases more than in accordance with Boyle's 

 law, that is pv decreases as p increases. To the left, the substance is a 

 liquid, and pv increases as p increases. 



