The atomic constants deduced from the study of a certain number of compounds are useful for calculating the molecular refraction of other substances. In general, the theoretical number thus obtained agrees with that found by experiment. More than any others, Landolt and Briihl have contributed to our knowledge in this direction.
The determination of the molecular refraction of a substance is extremely useful for ascertaining or confirming the chemical constitution. Thus the molecular refraction of benzene indicates the presence of three double linkages in agreement with KekuU's formula. Geraniol has the formula C 10 H 18 O and the characteristic properties of a primary alcohol. Its refractive index has been found to be 1*4745 for yellow light, from which we calculate the mole- cular refraction 48*71. The sum of the atomic refractions according to the preceding table is :
The difference between this and the experimental result is 48*71 — 45*375 = 8*335, which is very nearly twice the value for a double linkage (1*707 x 2 = 3*414). Geraniol thus appears to contain two double linkages; now, an alcohol C, H 18 O containing two double linkages cannot contain a ring and must therefore belong to the fatty series — a conclusion supported by the chemical character of the substance.
Bemarks
1.. The meaning of the factor (n — 1) in Gladstone and Dale's formula is easily understood. If we recall that
ami v n = -. — =t — , sinr v }
where v and t*, denote the velocity of transmission of the
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