OF GOLD (AND OTHEE METALS) TO LIGHT. 
161 
paration be left for six or eight months, a part will still remain suspended. Even in 
these portions, however, the diffused state of the gold is evident ; for where, as in some 
cases, the top to the depth of half an inch or more has become clear, it is seen that 
the ruby portion below is as a cloud sinking from it ; and in the part which has appa- 
rently been cleared from colour by the settling of the particles, the lens and cone of light 
still shows the few, or rather the fine diffused particles yet in suspension, though the 
protochloride of tin can show no gold in solution. The mould or mucus before spoken 
of, often collects the larger, heavier particles, and becomes of a dark blue colour; it 
may then be taken out by a splinter of wood, and being shaken in water, disengages 
the particles, which issue from it in clouds like the sporules from a ripe puff-ball. 
A gradual change goes on amongst the particles diffused through these fluids, especially 
in the cases where the gold is comparatively abundant. It appears to consist of an aggre- 
gation. Fluids, at first clear or almost clear to ordinary observation, become turbid ; being 
left to stand for a few days, a deposit falls. If the supernatant fluid be separated and left 
to stand, another deposit may be obtained. This process may be repeated, and whilst 
the deposition goes on, the particles in the fluid still seem to aggregate ; it is only when 
the fluid is deprived of much gold that the process appears to stop. Even after the fluid 
has attained a fine marked ruby tint, if allowed to stand for months in a place of equable 
temperature, the colouring particles will appear m floating clouds, and probably the 
aggregation is then still going on. That the particles of gold when they touch each 
other do in many cases adhere together with facility, is shown in many experiments. 
In order to test this matter mechanically, I gave much agitation to a dense ruby fluid, but 
did not find it cause any sensible change in the character. AVhen gold particles of a 
much larger size were agitated in water, they did cohere together, and the fluid, which 
requhed a certain time for setthng at the beginning of the experiment, settled in a much 
shorter time at the termination. 
If these fluids be examined generally their appearances differ, not merely under 
different circumstances, but also under the same circumstances, though they always con- 
sist of a colourless liquid and diffused particles of gold. A certain fluid in a bottle or 
glass, looked at from the front, i. e. the illuminated side by general daylight, may appear 
hazy and amethystine, whilst in bright sunlight it will appear light brown and almost 
opake. From behind, the same fluid may appear of a pure blue in both lights, whilst 
from the side it may appear amethystine or ruby. These differences result from the 
mixture of reflected and transmitted lights, both derived from the particles, the former 
appearing in greatest abundance from the Hont or side, and the latter from behind. The 
former is seen by common observation in a purer state if a black background be placed 
behind the fluid ; when a white background is (here, much of the transmitted light from 
that source comes to the eye, and the appearance is greatly altered. A mode of observing 
the former by a strong ray of light and a lens has been already described ; but even in 
that case some effects of transmitted hght are observed if the focus is thrown deep into 
the fluid ; and it is only the particles near the surface, whether illuminated by the base 
MDCCCLVII. z 
