322 MESSRS. C. CUTHBERTSON AND E. B. R. PRIDE AUX 



concordant than those obtained in Paris, and suggested, as in that case, that the 

 current of fluorine was not sufficiently rapid to displace completely the air in the 

 coolers and tubes. 



It was, therefore, decided to liquefy the gas as it was produced, and, when sufficient 

 had been collected, to allow it to boil off rapidly through the prism. This was done 

 with the arrangement shown in fig. 1, but the results showed no improvement, and it 

 seemed probable that some oxygen, either from the air or some other source, was 

 condensing with the fluorine, the boiling-points of the two elements being very nearly 

 identical. 



These experiments led to the conviction that it was practically impossible to obtain 

 fluorine in a state of absolute purity.* And since no means could be devised for 

 removing the gases by which it was accompanied without affecting the fluorine, it 

 was decided to have recourse to an analysis of the mixture of gases whose refractivity 

 was observed, and to correct for the impurities detected. For this purpose the prism 

 method was found unsuitable and JAMIN'S refractometer was substituted for it. 



The plan at first adopted was to displace the air in the refractometer tube by a 

 current of fluorine, counting the interference bands as they passed across the field, 

 owing to the change of refractivity. When a steady state was reached the tube was 

 disconnected from the source of fluorine and its contents collected over dry mercury, 

 by filling it with mercury from a reservoir. t It was anticipated that the fluorine, on 

 being bubbled through mercury, would instantly be absorbed as fluoride, and that 

 the residual gases could be measured and analysed. In a second set of experiments 

 glass tubes were used, and the residuals were collected over a standard solution of 

 soda. Here the reaction expected was the absorption of fluorine and production of 

 half its volume of oxygen, according to the equation 



2F 2 + 4NaOH = 4NaF + 2H 2 + O 2 . 



The principal figures connected with these experiments are given in the following 

 table : 



* Even in his most recent density experiment M. MOISSAN, after a prolonged trial with a current of 

 5 amperes, found 5 4 78 cub. centims. of nitrogen still left in a density bulb of volume 159-2 cub. centims., 

 or 3| per cent, of impurity. ' C. R.,' 138, p. 731, 1904. 



t A new refractometer tube was made for each experiment, when copper tubes were used. 



