195 
be investigated in a cuvette of 5 mm. in polarised light. It was. 
therefore, diluted with an equal volume of water and now gave 
between two crossed nicols on stirring, very plainly an illumination 
of the field. 
Microscopically were visible apparently round particles of about 
1 mw section in a strong Brownian movement. With some which 
were a little larger it was plainly visible in these rotations that 
they were flat. Evidently we are dealing here with the small hex- 
agonal mother of pearl-like glittering plates, which on crystallisation 
from warm gelatin-free solutions can be easily obtained in a larger 
dimension. 
6b. Pb-acetate and KI were mixed in the same proportion and 
the same dilution with this difference, however, that the gelatin 
solution was now 0.3°/,. The solution was warmed a little, so that 
the originally yellow amorphous turbidity dissolved clear and 
colourless and the liquid was then cooled. After a quarter of an 
hour the solution was greenish-yellow opalescent with incident light, 
brown with transmitted light. After the lapse of 6 hours the turbidity 
had become somewhat stronger, but no deposit had formed yet; 
also none after 20 hours and on filtering the liquid passed unchanged 
through the filter. After 5 weeks a portion had subsided but the 
supernatant liquid had still the same appearance as the 6 hours old 
colloidal suspension. | 
At first the solution gave no silky lustre on stirring, but did so 
after half an hour. Between crossed nicols it gave on stirring a 
bright illumination of the field. When streaming through a tube 
with rectangular section, placed between the 2 crossed nicols the 
field became luminous when the direction of the stream made an 
angle of 45° with that of the direction of polarisation of the nicols. 
When it was parallel therewith the field remained dark. 
Although the phenomenon was very much less strong than with 
V,O,-sol, the PblI,-sol is still essentially of the same nature; the 
streaming column of liquid behaves like a birefractive crystal, of 
which the directions of extinction rest parallel and perpendicular 
to the direction of the stream. 
Microscopically, nothing could be distinguished. The ultramicroscope 
exhibited very many small particles with a strong Brownian move- 
ment, yellow, brownish, red or of a more blue colour. Their light 
intensity varied very much, sometimes they suddenly dived in the 
field, and reappeared again. They made a strong impression of little 
dises toppling over their side. 
As we now know that the PbI, erystals, on addition of increasing 
13* 
