Page:Outlines of Physical Chemistry - 1899.djvu/94

 74 OUTLINES OF PHYSICAL CHEMISTRY

��Refractive Power op Liquids

It is well known that different coloured rays of light are refracted to an unequal extent and that white light suffers dispersion. It is, therefore, necessary to study the index of refraction of a liquid by means of a monochro- matic light, and for this purpose the yellow light of a sodium flame (d ray in the solar spectrum) is most often used, but sometimes also the red light of a hydrogen flame (c ray in the solar spectrum).

The index of refraction of a liquid varies with the tem- perature, the pressure, and in general with all the conditions which influence the density of the liquid or of the air. Attempts have been made to express the refractive power by a formula independent of these variations. Newton for theoretical reasons gave the formula — 5 —.

a

The division by d is carried out in order that the

refractive powers would be expressed as if all the liquids

had the same density as water. Gladstone and Dale have

shown that Newton's formula is inadmissible, and that the

expression — =- for the refractive power gives very much

more constant values. The formula is better adapted than any other to express the refractive power of solutions and of mixtures. If we denote by n 9 n u and n 2, and d, d lt and d 2 the indices of refraction and the densities of a solution, the dissolved substance and the solvent respectively, then the equation

100 n -=l = p n A=l + (100 -p) ^-pl

indicates the refractive power of a solution containing p parts of dissolved substance and consequently (100 — p) parts of solvent. And as n, d, n 2, d if may be experimen- tally determined, we are in a position to calculate

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