of Gold and other Metals to Light, 533 
days :—being boiled with zine filings it does not change; and 
even when dilute sulphuric or hydrochloric acid is added to evolve 
nascent hydrogen, still the ruby character undergoes no alteration. 
Strong sulphuric, or nitric, or hydrochloric acid do not alter it 
whilst cold; but when warmed, the first causes the gold to se- 
parate as dark aggregated metallic particles, and the two latter 
gradually cause the change to amethyst and blue formerly de- 
scribed. Chlorine, or a mixture of hydrochloric and nitric acids, 
dissolves the gold, the ruby colour disappears, and the ordinary 
solution of gold is produced. In all these cases the ruby gold 
behaves exactly as metallic gold would do with the same agents, 
and quite unlike what would be expected from any possible com- 
bination of oxygen and gold. 
In some of these jellies the ruby particles are so determinate 
as to give the brown reflexion by common observation; in others 
they are so fine as to look like ruby solutions, unless a strong 
sunlight and a lens be employed; and the impression again 
arises, that gold may exist in particles so minute as to have little 
or no power of reflecting light. Ruby particles of extreme fine- 
ness, when present in small amount in water, appear to remain 
equally diffused for any length of time; if in larger amount, 
that which settles to the bottom will remain for weeks and months 
as a dense ruby fluid, but without coming together: both cir- 
cumstances seem to imply an association of the particles of gold 
with envelopes of water. Many circumstances about the ruby 
jellies imply a like association with that animal substance, and 
many of the stains of gold upon organic substances probably in- 
clude an affinity of the metal of the like kind. 
Relations of Gold (and other metals) to polarized Light. 
It has been already stated, that when a ray of common light 
passes through a piece of gold-leaf inclined to the ray, the light 
is polarized. When the angle between the leaf and the ray is 
small, about 15°, nearly all the light that passes is polarized ; but 
as the leaf is really very irregular in thickness, and ill-stretched 
as a film, parts inclined at different angles are always present at 
once. The light transmitted is polarized in the same direction 
as that transmitted by a bundle of thin plates of glass, inclined 
in the same direction. The proportion of light transmitted is 
small, as might be expected from the high reflective power of 
the metal. The polarization does not seem due to any con- 
strained condition of the beaten gold, for it is produced, as will 
be shortly seen, by the annealed colourless leaf-gold, and also by 
deposits of gold particles; but is common to it with other un- 
crystallized transparent substances. It would seem that a very 
small proportion of the gold-leaf can be occupied by apertures, 
