150 
DE. FAEAEAT ON THE EXPEEBIEXTAE EELATIOXS 
that it is beaten amongst tissues softer than itself, and made up uith it into con- 
siderable masses. The greening effect of pressure occurs -nith 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 size of the particles or their quantity, but 
i-ather upon the relative dimensions of the particles in the direction of the ray and tiuns- 
■verse 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 dii’ection and extent of the '\ibrations of the ether. 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 loAver than that requii’ed 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 m chlorine gas. AWien examined 
by a lens or an ordinary microscope, the leaf seems to be as continuous as in its original 
state ; the finest hole, or the finest line draum by a needle-point, appears only to prove 
the continuity of the metallic film up to the very edges of these real apertiues. 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, riewed in the micro- 
scope, assumes a mottled appearance, as if a retraction into separate parts had occm-red. 
At a still higher temperature this effect is increased ; but the heat, whether applied in 
the muffle or by a blowpipe, which is necessary to fuse the metal and make it niii 
together in globules, is very much higher than that which causes the fflst change of the 
silver: the latter is, in fact, below such a red heat as is just risible in the dark. AMiat- 
ever the degree of heat applied, the metal remains as metallic silver diu'ing the whole 
time. When many silver leaves were laid loosely one upon another, rolled up into a loose 
coil, introduced mto a glass tube, and the whole placed in a muffle and heated carefully 
for three or four hours to so low a degree that the glass tube had not been softened or 
deformed, it was found that the silver-leaf had sunk together a little and shaped itself 
in some degree upon the glass, touching by points here and there, but not adhering to 
it. But it was changed, so that the light of a candle could be seen through forty thick- 
nesses : it had not run together, though it adhered where one part touched another. It 
did not look like metal, unless one thought of it as divided dead metal, and it even 
