276 
MESSRS. THOMAS MARTIN LOWRY AND PERCY CORLETT AUSTIN 
aqueous solutions of potassium, sodium and ammonium tartrates (‘ Mem. Acad Sci.,’ 
1838, vol. 16, pp. 304-317) showed that these agreed very closely with those produced 
by quartz plates of suitable thickness, and with those calculated from Biot’s law. A 
similar conclusion was reached by Ivrecke, who measured the rotatory powers of the 
potassium, sodium and ammonium salts over the range from 0° to 100° C., and concluded 
that “ the tartrates examined obey the laws of Biot ” (‘ Arch. Neerland.,’ 1872, 
vol. 7, p. 114). Krecke’s figures actually show small regular deviations in the case of 
potassium and sodium tartrates, and very marked deviations in the figures for ammonium 
tartrate and for Rochelle salt. Thus his figures for [a] X 2 are as follows :— 
Tartaric acid 50% 
„ . „ 50% 
Potassium tartrate 20%.. 
Sodium tartrate 20% 
Ammonium tartrate 10% 
Rochelle salt 20% 
C. 
D. 
E. 
b. 
F. 
o 
0 
2,427 
2,227 
1,602 
1,533 
1,331 
100 
5,792 
5,289 
4,942 
4,928 
4,646 
0-100* 
9,407 
9,245 
9,166 
9,285 
9,479 
0-100* 
9,067 
8,977 
8,863 
8,798 
9,166 
25 
13,376 
12,860 
11,948 
12,081 
12,608 
0-100* 
8,179 
7,944 
7,542 
7,508 
7,673 
The figures for tartaric acid, which are given for comparison, show the sensitiveness 
of the free acid to changes of temperature, as well as its complete liberation from the 
requirements of Biot's law. Thus the specific rotatory power of potassium tartrate for 
sodium light decreases by 3 per cent, only, from 27-223° to 26-415°, on raising the 
temperature from 0° to 100° C. ; that of the sodium salt increases by 2 per cent., from 
25-702° to 26-256° between 0° and 75° C., and then diminishes again to 25-211° at 
100° C.; whilst in the case of Rochelle salt there is a regular increase of 9 per cent, 
from 21 -820 at 0° to 23-993° at 100°. The rotatory power of the free acid, however, is 
practically three times as great at 100° as at 0° C. 
Our own observations of the rotatory power of the tartrates of the alkalis are set out- 
in Tables VI., VII., VIII. and IX. As in the two long series of readings of tartaric 
acid, filtered solutions (made up volumetrically) were used to facilitate the reading of 
the more difficult wave-lengths ; their exact concentrations were then determined by 
comparing the readings for Hg 5461 with those made with unfiltered solutions, prepared 
gravimetrically from samples of the various salts in which the proportion of moisture 
had been checked by analysis. In the case of the ammonium salt, which decomposes on 
drying even at moderate temperatures, standard solutions for the determination of 
concentrations were made by adding the calculated amounts of a titrated solution 
of ammonia to weighed amounts of tartaric acid in a stoppered flask and diluting the 
solution to a known weight. Densities of all the solutions were also determined. The 
data used in these calculations are shown in Table X., where also are given a number 
% Average values. 
