196 
quantities of gelatin, are obtained in steadily decreasing dimensions, 
we may assume that the ultramierones in this gelatin-rich solution 
are again small PbI, erystals, hence, small plates of an optic mona- 
axial crystal of which the optic axis stands perpendicular to the plane 
of this plate. . 
_ On streaming, these plates will arrange themselves parallel to the 
direction of the stream. 
The optic axis then stands perpendicular to the direction of the 
stream. It is evident that a column of these particles so directed 
will behave optically active and will extinguish parallel to or per- 
pendicular to the direction of the stream. 
HgCt. Solutions of NaCl and somewhat acidified HgNO, mixed 
in such proportion that the final solution contained 0.001 gram- 
molecule of HgCl per Litre, gave a nice silky suspension of HgCl 
needles of which the dimensions were about 0.5 at 10u. These 
crystals belong to the tetragonal system and, according to GROTH *) 
are extraordinarely strongly birefractive. 
By addition of some gelatin their dimension could be lessened. 
With 0.3°/, gelatin and 0,01 norm. HNO, a solution was obtained of 
yellowish-brown colour with transmitted light and mitky bluish-white 
with incident light. It could be filtered without undergoing change and 
gave between crossed nicols a strong illumination of the field when 
being stirred. On running through a tube with rectangular section 
the field became luminous when the direction of the stream made 
an angle of 45° with the direction of polarisation of the nicols, but 
not if it ran parallel to one of them. The streaming: column of 
liquid thus again behaves like a birefractive crystal of which the 
directions of extinction coincide with and stand perpendicular to 
the direction of the stream. 
With the ultramicroscope elongated particles showing a_ peculiar 
flashing of light were very plainly visible; they dived suddenly in 
the field and reappeared and altogether made the impression of 
mall spillars tumbling over their top. The apparent dimension of 
these particles varied from '/, >X<'/, to '/, 3 u. 
The appearance of these particles altogether resembles that of the 
small HgCl erystals which could be obtained in a still microscopi- 
cally visible dimension (up to 0.25 >< 1u) by addition of somewhat 
less gelatin and which in a similar way displayed their Brownian 
movement. 
As with the HgCl the size of the particles can be varied at will 
1) GrotH, Chemische Krystallographie, (1906) I, 214. 
