ON OPTICAL ROTATORY DISPERSION. 
285 
magnitude of the dispersion-constant O'0477 did not present the same grounds for 
suspicion as in the case of the tartrates of the alkali metals. In view, however, of our 
experience with these tartrates, we considered it necessary to make a series of measure¬ 
ments at least as complete and as extensive as that which had disclosed the real com¬ 
plexity of the rotatory dispersion in sodium tartrate. The solution used for these 
measurements was made up to be approximately M/60 or 5-53 grams of tartar emetic 
per 100 c.c. ; a comparison with the rotations of unfiltered solutions made up gravi- 
metrically, Table X., gave the concentration as 5-32 grains per 100 grams of solution 
of density 1-0368, or 5-516 grams per 100 c.c., and this figure was adopted in calculating 
the molecular rotations. The rotations of this solution are set out in Table XVI., 
which gives the results of visual readings for 18 lines and photographic readings for five 
lines, as compared with 16+6 readings in the case of sodium tartrate. Although the 
observed rotations are multiplied by 10 in order to convert them into molecular rota¬ 
tions, their agreement with the calculated values is very satisfactory ; even in the 
photographic region the largest error is only 1 per cent., and all the errors appear to 
be distributed quite casually, without any of the long series of positive or negative errors 
observed in the case of the alkali-tartrates. We therefore conclude that the rotatory 
dispersion in tartar emetic is definitely “ simple ” in character, and that in this compound 
we have for the first time succeeded in eliminating completely that complexity which is 
so strongly developed in the acid and its esters. 
Similar remarks may be applied to the alkaline solutions of tartar emetic. The 
preliminary series of observations for seven lines in the visible spectrum showed that 
[M]X 2 increased from —105-8 at wave-length 6438 to —133-3 at wave-length 4358, 
but that the dispersion could be represented satisfactorily by a simple formula. The 
simplicity of the rotatory-dispersion was, however, confirmed by a series of 17 visual 
readings as shown in Table XVII. Photographic readings are not included, as the 
solutions are not stable and show a marked diminution of rotatory power on keeping ; 
the visual readings could be taken before any serious alteration had occurred, but the 
photographic readings would have occupied too much time for this to be done successfully. 
10. Compounds of Arsenic and Bismuth. 
The arsenyl compounds H(AsO)C 4 H 4 O h , K (AsO) C 4 H 4 0 6 , Na(As0)C 4 H 4 0 6 , corre¬ 
sponding with tartar emetic were examined by Landolt (‘ Ber. Deutsch. Chem. Gesell., 
1873, vol. 6, p. 1077, and “ Optical Rotatory Power,” tr. 1902, p. 553). No measure¬ 
ments of dispersion were made, but the data showed that the arsenyl compounds have 
much the same rotatory power as the simple alkali-salts and do not exhibit any of the 
special qualities of tartar emetic. Grossmann ( loc. cit.) states that the addition of 
sodium hydroxide to sodium arsenyl tartrate appeared to cause complete decomposition 
of the salt into neutral disodium tartrate and optically-inactive sodium arsenite. Our 
own observations on the rotatory dispersion in an alkaline solution of the arseno-tartrate, 
