458 M. Haagen on the Refractive Indices 



we obtain 



XZZ' v 



and, introducing the values for the wave-lengths expressed in 

 hundred-thousandths of a centimetre, 



X a =6-564, andX y = 4-339 (Pliicker), 



Py — fig ._ B 



0-029906' ^ 18-227 



The specific gravities of the liquids were also accurately deter- 

 mined for the temperature 20°, as compared with the specific 

 gravity of water for the same temperature. The boiling-points 

 of the substances were determined by means of accurate thermo- 

 meters, and were corrected for that part of the mercurial thread 

 which projects out of the retort. 



The author then gives the particulars of a number of his ex- 

 periments, from which the refractive equivalents of a number of 

 elements may be calculated from the refractive equivalents of 

 their compounds. For calculating the refractive equivalent, the 



author used the formula P , , in which P is the atomic weight 



and is the specific refractive equivalent. If R is the re- 



fractive equivalent of a compound, r, r, . . . those of its elements, 

 and m, m 1 , . . . the number of atoms as expressed by the chemical 

 formula, the relation holds good, 



R = mr + m!r' -f m"r n + • • • , 



from which the refractive equivalent of any element may be cal- 

 culated, knowing those of the others as well as R ; for 



R — m'r 1 + m u r n 



r= — . 



m 



Landolt has calculated the refractive equivalents of the ele- 

 ments carbon, hydrogen, and oxygen from the refractive equiva- 

 lents of the liquid compounds, and assuming the refractive index 

 /£ a , and the atomic weights H = l, 0=16, €=12, has obtained 

 the following values : — 



H a =l-30; O a =3-00; € a =l'50. 



By the aid of these numbers the refractive equivalents of the ha- 

 loids may be calculated from the liquid compounds they form 



