optical Properties oj Phospliorus. 



31 



of the violet to be H, it gives — 

 Length of spectrum . 

 Dispersive power . . 



A'H-/^A=0-2038 



fJ'D- 



• 1 



= 0-1781 



The spectrum seen through bisulphide of carbon is not half 

 so long as this, fiu — f^A being at the same temperature only 

 0-0906. The dispersive power is 01460. Oil of cassia is also 

 largely exceeded in this respect by phosphorus, and the only- 

 substances which are reputed to be its rivals are realgar and 

 chrolmate of lead ; but to them have been assigned the scarcely 

 credibe dispersive powers of 0-255 and 0-3. Strange to say, 

 the measurements of the spectrum seen through phosphorus, 

 which have been hitherto published, assign to it a length little 

 exceeding that of the bisulphide of carbon spectrum, and con- 

 sequently a dispersive power considerably less. 



Liquid Phosphorus. — The determination of the refractive and 

 dispersive power of phosphorus in a liquid condition was attended 

 with some difficulty, not merely on account of the inflammability 

 of the melted element, but also because it attacked so readily 

 the cement of the hollow prism and caused it to leak. After 

 several trials, plaster of Paris was found to be an efficient means 

 of retaining it. The following are the indices of refraction at 

 35° C. :— 



These numbers indicate a considerable diminution both in the 

 refractive and in the dispersive power. The change from the solid 

 to the liquid state is also attended with a considerable diminu- 

 tion of density ; and the ratio between the density and the mean 

 refraction, fx,^ — !, is not far from being the same in the two 

 conditions. Thus, in the paper already referred to, the index of 

 refraction for the orange ray just before the line D, was found 

 to be for solid phosphorus at 35° C, 2-1168 ; and for the same 

 specimen when melted, but at the same temperature, 20709. 

 The specific gravity of phosphorus in the two states at about 

 35° C. has been subsequently determined. The following Table 

 shows the ratio obtained by dividing the specific gravity by the 

 mean refraction, that is, the refractive index minus unity : — 



