ON THE REFRACTIVE INDEX OF GASEOUS FLUORINE. 



321 



In all these trials a singular fact was observed. The index slowly decreased to a 

 minimum, and, after remaining steady for a few minutes, retrograded by several 

 points, indicating the presence of an increased proportion of some gas of higher 

 refractivity. This effect was observed in nearly every subsequent experiment 

 performed with the prism, and its significance will be referred to hereafter. 



But, in spite of the concordance between the lowest values obtained on the first 

 and second days, these experiments could not, for several reasons, be regarded as 

 satisfactory. All previous estimates of the refractivity of fluorine, based on the 

 refraction equivalent, point to a much lower value than 227 ; and, though this 

 expression cannot be relied upon to give a very close approximation, its agreement 

 with the refractivity is usually fair, and there is no other instance of so wide a 

 discrepancy between the two as these figures would show. 



In the second place, it was not certain that the current of gas employed, which 

 was at the rate of l to 2 litres per hour, would completely displace, from the train 

 of purifiers, a volume of 100 cub. centims. of air, whose density is not far removed 

 from its own. And, thirdly, the retrograde motion of the index after reaching a 

 minimum, and subsequent slight variations, definitely proved that the contents of the 

 prism were not homogeneous. 



For these reasons the authors determined to undertake a further investigation of 

 the subject, using the apparatus with which Mr. PRIDEAUX had, by this date, 

 succeeded in obtaining fluorine, by the method of M. MOISSAN. The form of the 

 electrolytic tube employed may be seen in fig. 1. It was kept cool by means of a 



Fig. 1. 



mixture of alcohol and solid carbonic acid, in such proportions as to form a paste. 

 The current ranged from one to two amperes. In two or three minutes the voltage 

 usually ran up to its steady value, and, soon after this, a piece of blotting paper, 

 wetted with alcohol, when held near the exit tube gave abundant fumes of HF 

 and then burst into flame. When this was observed the U-tube was connected to 

 the train of purifiers and prism or refractometer tube by means of a well-fitting 

 platinum junction. 



Many experiments were made with this apparatus, but the results were not more 



VOL. ccv. A. 2 T 



