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on stirring. The ultramicroscopic image showed clear round particles 
on a faint opalescent, optically non-resolvable back ground. On 
running through a tube with rectangular section (interior 82 mm.) 
at an angle of 45° placed between two crossed nicols in a ray of 
Na-light, absolutely no jlashing could be noticed; the field remained 
quite dark. 
A part of the solution was now beated for 4 hours on the water- 
bath (Dé). It then, on stirring, exhibited the silky lustre. Ultra- 
microscopically very delicate needles were visible. When running 
through the tube placed at an angle of 45° between crossed nicols, 
the field became very strongly luminous; on placing it parallel to 
the direction of the polarization of one of the nicols the field, during 
the streaming remained dark. 
The freshly prepared sol is therefore not birefractive. The pheno- 
menon only sets in and increases in strength with the formation and 
the growth of the ultramicroscopic needles. 
The solution D/ was again heated for 12 hours on the water- 
bath. The particles were increased in dimension but not micro- 
scopically visible. 
In a five months old fairly concentrated sol large ultramicroscopic 
needles were visible, which, however, were out of reach with the 
ordinary microscope. 
Summarising it appears in a very convincing manner that a slow 
growth of the V,O, ultramicrones is observable. This, however, is 
so trifling that we have hitherto not succeeded in obtaining particles 
of microscopically observable’ dimension. 
I. Birefractive sols of erystallisable substances. 
The peculiar silky lustre exhibited by old V,O, sols on stirring 
is also noticed in the formation of different crystalline precipitates. 
Some of these suspensions such as of BaSO,, BaSil’,, SrSO, mica, 
kaolin, soap, Hg,Cl, and Pbl, were now tested as to double refrac- 
tion, of these the two last gave a positive result. 
In order to succeed, the precipitate must, however, be very finely 
divided and not deposit so that the suspension has a colloidal 
character. It is obtained in that condition by allowing it to form 
in a very dilute solution and in the presence of a protective 
colloid. 
Pbl,. a. Lec. of Ol n. Pb-acetate + 8 ec. of 0.05°/, gelatin + Ice. 
of 0,1 n. KI were added together. There is formed an orange yellow 
suspension of a beautiful silky precipitate. This was too turbid to 
