REFRACTION AND DISPERSION OF THE HALOGENS, HALOGKN ACIDS, ETC. 17 



used by BERTH ELOT. The average yield was 6 per cent, by volume, but on one or 

 two occasions it reached 10 per cent. We failed to identify the causes which 

 produced these higher yields, and were unable to rejx-iit them, hut succeeded in 

 obtaining between f> ;tnd 7 per cent, with fair regularity. 



.Methods. As in the case of other gases, tin- work was <li\'ul.-<l into two part* 

 (l) the determination <>f the refractivity for a single wave-lengtli (tin- green mercury 

 line), and (2) the measurement of the dispersion in the visible spectrum relatively to 

 this value. 



For the measurement of the refractivity two methods were employed. In the first 

 of these the quantity of ozone present was estimated by destroying the ozone by heat, 

 and measuring the increase of the gas in volume. In the seco/id, the ozone was 

 estimated chemically by bubbling the mixture of gases through a solution of potassium 

 iodide, and titrating with thiosulphate of soda. 



As the results of the enquiry were remarkable the following details may be of 

 interest : 



Dry oxygen, prepared by heating permanganate of potash, and stored in a gas 

 holder over mercury, was led through an ozoniser into the interferometer tube, whirl i 

 was previously evacuated. The interference bands which crossed the field were 

 counted till atmospheric pressure was reached. The pressure was then read by con- 

 necting the apparatus with a mercury manometer filled with oxygen and separated 

 from the ozonised gas by a long capillary tube. The temperature of the water bath 

 was observed and the tap which led to the refractometer tube turned off. Having 

 again evacuated the connections the gas in the refractometer tube was allowed to 

 flow slowly into the pump, passing through a spiral of fused silica heated to redness, 

 which effectually destroyed the ozone. From the pump it was transferred to another 

 gas holder over mercury and thence again allowed to flow into the refractometer tube, 

 where its temperature and pressure were again measured. If V, is the volume of 

 the ozonised, and V 2 that of the deozonised oxygen, the percentage of ozone is given 

 by V, Vi = afVj/200. In the present case V, was about 150 c.c., so that if x = 8 the 

 total increase of volume is 6 c.c. In order to determine the value of the refractivity 

 to 1 per cent, it is therefore necessary that the total error in pumping the gas round 

 the cycle should not exceed '06 c.c. In practice this accuracy was not quite attained. 

 It was necessary to grease stopcocks with a mixture of pure paraffin and vaseline, 

 which will not hold a vacuum indefinitely ; while, in order to destroy the ozone, the 

 gas had to be pumped through a spiral 12 inches long of fine capillary bore, which 

 made it difficult to evacuate the last traces from the connections. It was also 

 necessary to know the refractivity of the oxygen very accurately, since an error in 

 this figure is multiplied in the ratio of 100 fx. After a sufficient number of trials had 

 been made to prove that our oxygen was approximately pure, its refractivity was 

 assumed to be that previously determined by us,* viz., /u 1 = '0002717. X 5461. 



* C. and M. CUTHBERTSON, ' Roy. Soc. Proc.,' A, vol. 83, p. 151, 1909. 

 VOL. OCX 1 1 1. A. D 



