189 
Chemistry. - “Birefractive colloidal solutions” by Prof. W. 
Reinpers. Communicated by Prof. J. BörsEKEN. . 
(Communicated in the meeting of May 27, 1916). 
H. Freunpuicu, H. DiesserLnorsT and W. LrONHARDT describe in 
the “Festschrift für Erster and Germer” ') a remarkable phenomenon 
observed with vanadium oxide sol. The reddish-brown, very perma- 
nent colloidal solution, which is quite clear with transmitted light 
exhibits with incident light, on stirring, silky schliers *) looking like 
a swarm of very minute crystals; at the same time it becomes birefrac- 
tive. If the solution is allowed to flow through a tube with rectangular 
section placed between two crossed nicols, the field remains dark when 
the direction of the stream is parallel to the direction of extinction 
of one of the nicols; it however becomes strongly luminous so soon 
as the direction of the stream makes a certain angle therewith. 
An elucidation of this phenomenon was given by the ultrami- 
croscope which instead of luminous points showed very slender 
elongated needles, or pillars. When at rest these will occupy an 
arbitrary position so that the solution is then altogether isotropic. When 
the liquid is stirred, these*needles will, however, arrange themselves 
with their axis in the direction of the movement. The particles are 
then directed and a column of liquid wiih all these particles simi- 
larly directed will be capable of behaving like an optie monaxial 
erystal whose optical axis coincides with the direction of the stream. 
A further investigation with convergent polarised light has comple- 
tely confirmed this conception. It further appeared that, not only 
by mechanical stirring or by streaming, but also by introducing a 
magnetic field or by cataphoresis, the liquid becomes birefractive. 
H. R. Kruyr ®) has been able to confirm by ultramicroscopic inves- 
tigation that cataphoresis is really associated with a directing of 
the particles. 
Whereas to the form of the particles and their being directed 
by external forces no further doubt need exist, this is by no means 
the case with the nature of the particles themselves. Are these anisotropic 
already, are they minute crystals, or may we suppose them to be 
isotropic and explain the double refraction by the unequal elasticity 
in different directions of the solution as an homogeneous whole? 
‘) Arbeiten aus den Gebieten der Physik, Mathematik und Chemie, Braunsch- 
weig 1915, 453. 
2, Prof. G. A. F. MOLENGRAAFF informed me, that the word schlier as a trans- 
lation of the German word Schliere has been used by R. A. DAty in his book 
“Igneous rocks and their origin” 1914 p. 448. 
8) These Proceedings Voi. XVIII. p. !625. 
