of Gold and other Metals to Light. 407 
direction which annealing takes away ; but when the gold is ex- 
amined by polarized light, there is no evidence as yet of such a 
condition, for the green and the colourless gold present like 
results ; and there isa little difficulty in admitting that such an 
nregular corrugated film as gold-leaf appears to be, can possess 
any general compression in one direction only, especially when 
it is considered that it is beaten amongst tissues softer than 
itself, and made up with it into considerable masses. The green- 
ing effect of pressure occurs with the deposited particles of 
electric discharges, and here it appears either amongst the larger 
particles near the line of the discharge, or amongst the far 
finer ones at a considerable distance. Such results do not 
suggest a dependence upon either the swe of the particles or 
their quantity, but rather upon the relative dimensions of the 
particles in the direction of the ray and transverse to that 
direction. One may imagine that spherical or other particles, 
which, being disposed in a plane, transmit ruby rays or violet 
rays, acquire the power, when they are flattened, of transmitting 
green rays, and such a thought sends the mind at once from the 
wave of light to the direction and extent of the vibrations of the 
ther. For it does not seem likely that pressure can produce 
its peculiar result by affecting the relation of the dimension of 
the particle to the length-dimension of a progressive undulation 
of light, the latter being so very much greater than the former ; 
but the relation to the dimension of the direct or lateral vibration 
of the particles of the ether may be greatly affected, that being 
probably very small and much nearer to, if not even less than, 
the size of the particles of gold. 
Silver-leaf, as usually obtained by beating, is so opake, 
as perfectly to exclude the light of the sun. When this is laid 
by water on plates of rock-crystal and heated in a muffle, it 
begins to change, at a temperature lower than that required for 
gold, and becomes very translucent, losing at the same time its 
reflective power: it looks very like the film of chloride produced 
when a leaf of silver is placed in chlorine gas. When examined 
by a lens or an ordinary microscope, the leaf seems to be as con- 
tinuous as in its original state; the finest hole, or the finest line 
drawn by a needle-point, appears only to prove the continuity of 
the metallic film up to the very edges of these real apertures. 
When pressure is applied to this translucent film, the compressed 
metal becomes either opake or of a very dark purple colour, and 
resumes its high reflective power. If a higher heat than that 
necessary for this first change be applied, then the leaf, viewed 
in the microscope, assumes a mottled appearance, as if a retrac- 
tion into separate parts had occurred. Ata still higher tempera- 
ture this effect is increased; but the heat, whether applied in 
