of Gold and other Metals to Light. 527 
until the gold is reduced, it will generally be found that the 
vapour has carried a portion of gold on to the neighbouring 
part of the glass, and that this part, when placed over a sheet 
of white paper, has the ruby tint. With the rock-crystal both 
ruby and blue parts are produced; and when the ruby parts are 
subjected to rock-crystal pressure, they become beautifully green. 
In the arts also glass is oftentimes coloured ruby by gold; I 
think that glass in this state derives its colour from diffused 
divided gold; and if either the ruby glass or the watch-glass be 
examined by a lens and the cone of rays, it will be seen that 
the colours are not due to any gold dissolved, but to solid and 
diffused particles. There is nothing in any of the appearances 
or characters, or in the processes resorted to to obtain the 
several effects, that point at any physical difference in the 
nature of the results; and without saying that gold cannot pro- 
duce a ruby colour whilst in combination or solution, I think that 
in all these cases the ruby tint is due simply to the presence of 
diffused finely-divided gold. 
Metallic character of the divided gold. 
Hitherto it may seem that I have assumed the various pre- 
parations of gold, whether ruby, green, violet, or blue in colour, 
to consist of that substance in a metallic divided state. I will 
now put together the reasons which cause me to draw that con- 
clusion. With regard to gold-leaf no question respecting its 
metallic nature can arise, but it offers evidence reaching to the 
other preparations. The green colour conferred by pressure, 
and the removal of this colour by heat, evidently belong to it as 
a metal; these effects are very striking and important as regards 
the action on light, and where they recur with other forms of 
gold, may be accepted as proof that the gold is in the metallic 
state. Although I do not attach equal importance to the fact 
already described, that gold-leaf frequently presents fine parts 
that appear to be ruby in colour, I am not as yet satisfied that 
they are not in themselves ruby; and if they should be so, it 
will be another proof by analogy of the metallic nature of other 
kinds of preparations eminently ruby. 
The deflagrations of gold wire by the Leyden discharge can 
be nothing but divided gold. They are the same whatever the 
atmosphere surrounding them at the time, or whatever the sub- 
stance on which they are deposited. They have all the chemical 
reactions of gold, being, though so finely divided, insoluble in 
the fluids that refuse to act on the massive metal, and soluble in 
those that dissolve it, producing the same result. Heat makes 
these divided particles assume a ruby tint, yet such heat is not 
likely to take away their metallic character, and when heated 
