CHANGE OF STATE LIQUID VAPOUR. 



173 



was not very large and it was found possible to keep its contents at a 

 uniform temperature. But above 50 a much larger bath would have 

 been required, and the difficulty of maintaining a uniform temperature in 

 a large mass of water is so great that Regnault abandoned this method for 

 the dynamical. 



(b) For temperatures below he still employed two barometers, side 

 by side (Fig. 96) ; but one barometer was bent round at the upper end, 

 and terminated in a bulb E, containing ice. This was surrounded by a 

 " freezing mixture," the temperature of which was taken by a thermo- 

 meter. The temperature of the rest of the apparatus being above that 

 of the freezing mixture, the pressure exercised by the vapour on the 

 mercury corresponded to the temperature of the 



freezing mixture for any tendency to rise above 

 this pressure would be checked by condensation 

 in E. The freezing mixture was liquid, and by 

 stirring, its temperature was kept uniform through- 

 out. Regnault was unaware of the fact that the 

 vapour-pressure of ice below differs from that 

 of water below at the same temperature, a fact 

 deduced subsequently from theory by Kirchhoff, 

 and later verified experimentally by Ramsay and 

 Young. He, therefore, made no attempt to measure 

 the vapour-pressure of water below 0. In order 

 to construct tables giving the most probable values 

 of the vapour-pressure for each degree of tempera- 

 ture, Regnault employed the graphic method, 

 plotting his results on a temperature and pressure 

 diagram, and then getting rid of irregularities or 

 discrepancies between different series by drawing 

 a continuous curve most nearly representing the 

 results. The curve, as drawn by Regnault, goes 

 continuously through 0, where the evaporating 

 substance changes from water to ice. This throws 

 doubt on the accuracy of his numbers about and 

 below 0. The water-vapour curve would no doubt 

 be continuous, but the ice-vapour curve meets it at 



a small angle at as represented in Fig. 97, theory showing that the 

 ice-vapour curve diverges from the water-vapour curve about -^ mm. per 

 degree. Ramsay and Young (Phil. Trans., ii., 1884, p. 461) have proved 

 that the difference exists by maintaining ice and water in two separate 

 connected vessels at a very low constant pressure. The temperatures 

 of the two vessels became steady at the points at which their vapour- 

 pressures were equal to this constant pressure. The ice was found to 

 be at a higher temperature than the water. 



(c) For temperatures above 50 Regnault employed the dynamical 

 method. The arrangement of the apparatus will be seen from Fig. 98. 

 A copper boiler, heated by a small furnace, is in connection with the 

 reservoir of air G, the pressure of which is indicated by the manometer 

 HK. Round the connecting tube at A is a wider tube, in which water 

 circulates as in an ordinary still. The boiler is heated till the water in 

 it boils, and the steam rises into the tube A. Here it is condensed, and 



FIG. 96. Vapour- 

 Pressure of Ice. 



