961 
red, ultramicr: green ete. With a very rare exception exhibiting 
Brownian motion, they all were immovable, therefore tied fast to 
the back-ground. 
The apparent size of these ultramicrons, hence their mutual distance 
was almost equal in the several compounds, there was at any rate 
no obvious connection between the variations in this apparent dimen- 
sion and tbe macroscopically investigated colour of the deposit. So 
this proves that the distance between the particles is about equal, 
if, at any rate the thickness of the film is not such, that more 
layers are lying one above the other. But the intensity of the light 
of the particles varies; with the red and the blue films it is fairly 
strong; with the yellow films it is on the whole but slight. 
Associating this with the theory of Rayreian ') we draw the con- 
clusion: that the dimensions of the yellow ultramicrons are smaller 
than those of the red and the blue ones. 
There is this essential difference between the coloured silver-film 
and the clear layer of salt that the latter is optically homogeneous, 
the former distinctly heterogeneous. 
The question arose whether this heterogeneousness was already 
present in the silver deposit in its original form, or whether it 
proceeded from the effect of the air as is the case with rock-salt. 
To obtain a solution to this question we again fused a side-tube 
with Canada Balsam to the silver-lamy as we had previously done 
with the salt-lamp. By gentle heating in high-vacuum the balsam 
was freed from the dissolved gases and when the deposit of silver 
had been formed it was carefully spread over it, so that part of it 
was covered by the Canada Balsam. 
On opening the lamp the colour of the deposit of silver that had 
been covered with Canada Balsam remained the same, whereas in 
the uncovered part a distinct change of colour set in. 
In ultramicroscopic investigation it appeared that the part covered 
with Canada Balsam also consisted of a network of ultramicrons. 
There was no marked difference between the apparent size of these 
ultramicrons and those of that part that had been exposed to the 
air. A sharp demarcation line between the two fields, such as had 
been observed with the rocksalt and which here too, was macros- 
copically quite perceptible on account of the variation in colour, 
was looked for in vain. The two parts imperceptibly passed into each 
other: the network itself showed no difference and only in the total- 
colour of the ultramicroscopic field, which had more of brown in if 
1) Phil. Mag. (4), 41, 107, 274, 447 (1871). 
