808 Mr. F. B. Young on the 



o 



should be removed. The earlier tubes distinctly showed 

 effects due to the presence of such traces. The observations 

 made with tube VII. may be taken as typical. When the 

 vapour jacket was first heated, the aniline vapour, as it rose 

 round the tube, produced a reflux condensation which, as 

 before explained, had the effect of collecting the bulk of the 

 gaseous impurity at the top of the tube, whilst the bell was 

 filled with comparatively pure vapour. The tube was then 

 maintained at a constant temperature of about 185 c C. The 

 meniscus was observed to rise slowly in the bell ; after 

 7 minutes it had risen about halfway, and after 20 minutes 

 about three-quarters of the way up the bell. The bell was 

 next raised above the liquid so that it was filled with vapour 

 from above. It was then lowered to the bottom, and the 

 meniscus was again observed. The meniscus now remained 

 at the mouth of the bell and had not risen at all at the 

 expiration of 30 minutes. 



Tube XIV. was heated simultaneously with tube VII., and 

 in both of the above tests the meniscus remained at the 

 mouth of the bell. It is evident, therefore, that the meniscus 

 in tube VII. had in the first case been driven upwards by the 

 partial pressure of the enclosed gas, while this partial pressure 

 was lacking in tube XIV. 



Theoretically, the meniscus should have slowly risen in the 

 bell of tube XIV. owing to the pressure of the liquid ether 

 upon the vapour in the bell. From the data given by 

 Ramsay and Young (9) may be calculated the difference in 

 temperature of the two menisci which is necessary to maintain 

 them in equilibrium at a given difference of level. The 

 difference of temperature is o, 0008 C. for 1 cm. difference 

 of level, or 0°0064 C. for the difference of level of 8 cm. 

 which existed in the tube. The difference was probably a 

 little greater than this, for very occasionally a bubble escaped 

 from the mouth of the bell, showing that slow vaporization 

 was in progress. Thus the tube acts as a delicate differential 

 thermoscope whose delicacy increases as the mean temperature 

 rises, since not only does the density of the ether decrease 

 with rise of temperature but the slope of its vapour pressure 

 becomes steeper. The difference of temperature persisted in 

 spite of all precautions taken. About one-tenth of this 

 temperature variation may be ascribed to the effect of the 

 hydrostatic pressure of the aniline vapour upon its own 

 tension; the remainder must be due to other causes. 



If the temperature of the tube was suddenly raised, the 

 meniscus quickly travelled a certain distance up the bell and 

 then remained practically stationary. Doubtless the vapour 



