Page:Text-book of Electrochemistry.djvu/189

Bender (4) used a solution of potassium chloride for comparison, and found the following refractive indices for the ${\displaystyle H_{\alpha }}$, D, ${\displaystyle H_{\beta }}$, and ${\displaystyle H_{\gamma }}$ lines of the spectrum:—

x.	${\displaystyle H_{\alpha }}$.	D.	${\displaystyle H_{\beta }}$.	${\displaystyle H_{\gamma }}$.
Refractive index for KCI.
1	1.3409	1.3428	1.3472	1.3505
2	1.3498	1.3518	1.3565	1.3600
3	1.3583	1.3603	1.3651	1.3689
Moduli of the refractive index M × 10-4 for
K, Cl	0	0	0	0
Na	2	2	2	2
½Cd	38	—	40	41
Br	37	38	41	43
I	111	114	123	131

From this we can calculate, for instance, the refractive index of a 2-normal solution of sodium bromide for light of wave-length ${\displaystyle H_{\alpha }}$:

${\displaystyle n=1.3498+{\frac {2\left(2+37\right)}{10,000}}=1-3576.}$

The experimental value is 1.3578. It should, however, be noted that the agreement is not always so good. Le Blanc (5) has shown that weak acids and bases do not fit into this additive scheme.

Magnetic Rotation of Solutions.—Jahn (6) found the following values for the power of salt solutions to rotate the plane of polarisation in a magnetic field. He determined the angle through which the plane of polarisation of sodium light was turned in passing through a column of water of definite length in a strong magnetic field. This angle was taken as equal to 100. Working under precisely the same external conditions, he determined the angle for normal solutions of various electrolytes. From this he subtracted