284 MESSRS. THOMAS MARTIN LOWRY AND PERCY CORLETT AUSTIN 
conform to Biot’s law since [M]\ 2 increases from —3-9 at Cd 6438 to —5-7 at Hg 
4358 ; and they cannot be expressed by the simple Drude formula a (A 2 —A 0 2 ) = const., 
since the line obtained by plotting 1 /a against A 2 has a perfectly definite though very 
slight curvature. 
The effects produced by the addition of a few molecular proportions only of alkali 
to aqueous solutions of potassium and sodium tartrates are shown in Table XV. ( a ) and 
(6). The molecular rotations are a little lower than in Tables VI. and VII., but the 
solutions are still dextrorotatory, and the dispersions, though certainly complex, can 
be expressed very closely by a “ simple ” formula, which, in the case of the sodium salt, 
is identical with Biot’s law ; the readings are, however, too small to reveal the 
complexity first disclosed in Table VI. (a). 
9. Tartar Emetic. 
Tartar emetic, K(SbO) C 4 H 4 0, ; , |HX>, although one of the most interesting of the 
tartrates, does not appear to have been examined by Biot. The rotatory dispersion in 
tartar emetic was measured, apparently for the first time, in 1872 by Krecke (‘ Arch. 
Neerland., 1872, vol. 7, p. 114), who observed a very close agreement with the require¬ 
ments of Biot’s law. Thus he gives for [a] A 2 the following numbers :— 
C D E b F 
At 0°. 52,405 51,253 50,966 51,308 53,142 
At 100° .... 48,436 45,193 45,796 45,851 49,307 
He calls attention to the fact that “ the specific rotatory power of tartar emetic is 
extraordinarily great, and diminishes with rise of temperature,” and includes this salt in 
his general statement that “ the tartrates examined follow the law of Biot.”* Even more 
interesting than the high rotatory power of the salt is the fact discovered by Grossmann 
(‘ Zeitschr. Physikal. Chem.,’ 1907, vol. 57, pp. 533-556) that when sodium hydroxide is 
added, the specific rotation of the salt assumes a large negative value, probably because 
“ in the alkaline solution an antimonyl alkali tartrate is present, in which the hydrogen 
atoms of the alcoholic hydroxyl groups are also displaced.” 
Our own experiments give no support to Krecke’s view that tartar emetic obeys 
Biot’s law. Thus the first preliminary series of observations gave for the product 
[M]A 2 values which increased progressively from 156-17 at wave-length 6708 to 203-85 
at wave-length 3917. But a “ simple ” dispersion formula [M] = 140-67/(A 2 —0-0477), 
based upon the readings for the two dominant mercury lines, showed an agreement that 
was very satisfactory, especially in view of the fact that the observed rotations had been 
multiplied by nearly 12 to convert them into molecular rotations. Moreover, the 
* Eor further observations on tartar emetic, see Long, ‘ Amer. Journ. Sci.,’ 1889, series (3), vol. 38, 
p. 264; 1890, vol. 40, p. 275. 
