965 
tion. W. O. Rage!) has carried out a very extensive physico-chemical 
research with this substance, while S. Srrvanovíc®) has made some 
crystallographic measurements. The very marked change of colour 
which oceurs here renders the phenomena very suitable for demon- 
stration. Repeating and extending the experiments of Rape I have 
been able to confirm his results in all respects. 
9. The pure salt was prepared in the following way: Pure 
thallium was dissolved in sulphuric acid, and the thallous sulphate 
after having been recrystallised three times, was dissolved in water. 
This solution was mixed with a solution of pure baryta in small 
excess. The sulphate of barium was filtered off, and the solution of 
thallous hydroxide neutralised by an aqueous solution of pieric acid. 
(The acid had been recrystallised thrice from alcohol). The thallous 
picrate formed always (independently of temperature) separates out 
in the form of yellow erystals. As soon as the temperature of the 
solution has fallen below 40°, red erystals also make their appear- 
ance and after remaining in contact with the solution fors some 
days (at room temperature) the whole crystalline deposit becomes 
red. The salt is thus recrystallised from water. 
ile 
( ee 
5 = 3.16) are monoclinic, while the yellow 
The red erystals (d 7 
9Ko 
ones de = 2.99) are triclinic. 
10. Rape has shown by means of solubility determinations in 
methyl and ethyl alcohol that the transition temperature: is 46°. He 
thought it would be impossible to determine the transition point 
dilatometrically in this case, in consequence of the strong retardations 
which occur here, but I have succeeded in fixing this point using 
the dilatometer after having abolished the retardation (adding some 
water). In this way I also found 46° to be the transition temperature. 
11. The red salt was thoroughly dried in vacuo over sulphuric acid 
for some weeks. Repeating and extending Rasr’s experiments I got 
the following results: 
1. If the dry red erystals are heated for 24 hours at 76° (that is 
30° above the transition temperature!) yellow spots are formed on 
the surface. 
2. If the dry red crystals are heated for some hours to 100° (that 
) Zeitschr. für physik. Chemie 38, 175 (1901). 
2) Zeitschr. für Kristallographie 37, 257 (1903). 
