738 Dr. C. Statescu on Dispersion of Carbon Dioxide 



also used a similar method in his well-known work on the 

 dispersion of gases. 



The following is the theory of my experiment. Let <f> be 

 the angle of the gas prism and D the angle of minimum 

 deviation. Then the refractive index (n) of the gas is given 

 by the formula : — 



Sin 2 D . D ,$ 



— cos — -f sin tp cot £ . 



2 , „ 2 _- 2 



sm 2 



Since D has only a small value for gases we may write 



n-l- F oot|. 



Taking the results of Mascart*, Benoitj, Chappuis and 

 Riviere J, and of Sutherland § on the relation between tempe- 

 rature, pressure, and refractive index, this value of n — 1 

 may be reduced to that at 0° C. and 760 mm. pressure by 

 the formula : — 



?i — 1=2" cot |U +«0 — r> 



where n is the refractive index at 0° C. and 760 mm., and 

 p' is the pressure of the gas in millimetres of mercury 

 measured at t° 0. and reduced, for greater accuracy, to the 

 value it would have had at 0° C. P. Chappuis' value of 

 u ( =0*0037 16) was taken. The angle of the prism, </>, was 

 y0° 2' 12"-9. 



In the actual experiment I measured not the absolute 

 deviation and the absolute pressure, but the change in 

 deviation produced by a definite change in pressure, and 

 substituted these values in the above formula to obtain the 

 refractive index. 



The carbon dioxide was taken from a cylinder. It had 

 been used in a previous experiment, so that I was able to 

 assume it had been freed from air impurities. The gas was 

 dried over calcium chloride and phosphorus pentoxide before 

 being admitted into the gas prism. 



The arrangements for the prism were as follows. A 

 tunnel of rectangular section 4*5 X 5 sq. cm. was cut through 

 a block of brass. The sides of the prism were formed by 

 two plates of rock-salt 18*58 mm. thick. These were placed 

 inclined to one another in the tunnel. Thus the brass itself 



* Mascart, loc. cit. t Benoit, Journ. de Phys. (2) viii. (1889). 



% Chappuis et Riviere, Ann. de Chim. et de Phys. (6) xiv. (1888). 

 § Sutherland, Phil. Mag. [5] ii. pp. 141-155 (1889). 



