REFRACTION AND DISPERSION OF THE HALOGENS, HALOGEN ACIDS, ETC. 15 



STEAM. 



Previous determinations on the refractivity of water vapour are given by DUFET 

 as follows : 



No one appears to have attempted the dispersion. 



Procedure. A weighed quantity of distilled water, sealed up in a thin capillary 

 tube, was introduced into the refractometer tube, which was then evacuated and 

 sealed off. On breaking the capillary by a jerk the tube was filled with vapour. 

 After adjusting the tubes between the mirrors of the interferometer the centre of the 

 tube containing the water was first cooled to a known temperature and then the tube 

 was heated till the whole of the water present had evaporated. To the number of the 

 bands read was added a proportionate number for the vapour present at the initial 

 temperature. 



In order to eliminate the errors of drift other experiments were made in which the 

 ends of the tube were kept near the maximum temperature required (about 140 C.) 

 and the centre of the tube gradually cooled to the temperature of ice. 



Refractivity. Experiments were made with four charges of water. The results 

 are given below : 



Kefractivity of STEAM. 



It will be seen that the second charge yielded results considerably lower than the 

 other three. The cause of the discrepancy was found to be the unequal distribution 

 of vapour between the main portion of the refractometer tube aiid the small 

 " appendix " left when the side tube is sealed off. When the temperature of the 

 ends is markedly higher than that of the middle (as it was in this series), the error 

 becomes considerable. Neglecting this experiment we take the mean of the other 



