Chemistry und Physics. 71 
the chemical relations of the substances. This quantity has been 
called the specific refracting power, and if we now multiply by the 
molecular weight of the substance we have in M (4S) the spe- 
cific refracting power of the molecule, or as it has also been called 
the refractive equivalent of the subs tance. In 1864 (Pogg. Ann., 
exxili, 595) Landolt mepidsligg that a difference of one atom of car- 
bon, hydrogen or oxygen, between two organic compounds cor- 
responded to a constant difference for each elementary substance 
the <a meee that the refractive power of the molecule was 
simply ee m of the refractive Salado of its siimaisenien atoms. 
For exam 
—1 
ig re Diff 
a 
Methyl alcohol CH,0O 13°17 5-41 
Aldehyde C,H,0 18°58 
Differing in composition by He. 
Ethyl alcohol C.H,O 20°70 
Differing in composition by Oi. 
Aldehyde C.H,O 18°58 2°53 
Acetic — CeH,O. 21°11 
TABLE ITI. 
Ta JF 
C 5:0 4°86 
H big 4 1°29 
.8) 3°0 2°90 
Of course these values are based on the determination of the 
index of refraction, and the density of each of the substances thus 
compared. The pS of density were readily reduced to 
sandare conditions which might be arbitrarily selected, but in 
of the different substances compared as by no means a matter 
of indifference which of the lines of the spectrum was selected for 
the ex of refraction. oO ments Bruhl deter- 
calculated according to Cauchy’s form nula the index of refraction 
ing t 
these calculated indices the values 7, are deri ived. ta thus 
reduced are regarded by Bruhl as furnishing the more , aacees 
