118 REPOET— 1886. 



Analogy of Vaporisation of Solid ivith Boiling of Liquid. 



Bamsay and Young ' have obtained I'esults for ice by the dynamical 

 method, which are conclusive on this point, that ice has definite tem- 

 peratures of volatilisation without fusing, and for each temperature a 

 definite pressure. In the dynamical method the substance is either 

 boiling or volatilising when its vapour is being formed at a temperature 

 at which the vapour has a pressure just equal to the external pressure. 



Ramsay and Young used two flasks, each having a thermometei-, 

 connected at their necks by a narrow glass tube ; the tube was provided 

 with a side tube and a cHp, by which it could be either closed against 

 the outer air or attached to a pump ; or, what was found more 

 efficacious for excluding every trace of air, well-boiled water was put into 

 the flasks and boiled down in them, so that the steam expelled the air 

 very completely from the apparatus when the clip was closed air-tight 

 after the thermometers had been inserted. 



On placing one of the two flasks in a freezing mixture, ice at a low 

 temperature was formed, some adhering firmly to the bulb of the ther- 

 mometer. 



"When this flask was put in boiling water and the other in a freezing 

 mixture what happened was this, that after a little time the bulb of the 

 second thermometer was covered with ice, and soon the two thermometers 

 showed the same temperature, which they kept so long as the temperature 

 of the condenser was not altered, and so long as the bulb of the ther- 

 mometer in the other flask was covered with ice. 



If the temperature of the condenser was changed the two thermometers 

 both soon showed the same lower or higher temperature, but the variation 

 of the temperature of the water-bath had no effect on the temperatures 

 of the thermometers — in other words, on the volatilising point. 



The fixed point at which the two thermometers agreed in any experi- 

 ment was the temperature at which the pressure of vapour from the ice 

 on the bulb of the thei-mometer in the flask in the water-bath was just 

 equal to the pressure of the vapour from the ice on the bulb of the ther- 

 mometer in the flask in the condenser. 



If the air is not completely expelled, or if a very little air is intro- 

 duced, the flask in the water-bath shows a higher temperature than 

 the flask in the condenser — for this reason, that the ice in the first flask 

 must have a pressure of vapour more than enough to balance the pressure 

 of ice- vapour from the condenser flask by the pressure of the air ; the 

 pressure of this small quantity of air will not vary much in any one set 

 of experiments, and therefore by means of the different temperatures of 

 the two thermometers in one experiment and the vapour pressures corre- 

 sponding to the two temperatures we can, by taking the difference 

 between these two pressures, get the jjressure of the air in the appai-atus, 

 which pressure, being allowed for in the rest of the experiments in the 

 series, there was found always a satisfactorily constant agreement between 

 the volatilising temperature and the condensing temperature ; and not only 

 that, but also that the higher temperature given when air was introduced 

 was the temperature at which the vapour-pressure from ice and the 

 pressure of the air were jointly equal to the pressure found by Regnault 

 as the vapour-pressure of ice at that higher temperature by the statical 



> Phil. Trans. Part I. 1884. 



