178 
DE. FAEADAT THE EXPEEDIEXTAL EELATIOXS 
which had been re-greened by agate pressure acted in like manner on the polarized ray, 
but the experiments were imperfect. 
A glass plate having gold-leaf on one part of it, had a second glass plate put over it 
and gummed at the edges. In the sulphide of carbon, therefore, it represented in one 
part a plate of air, and in another a compound plate of air and gold ; both acted in the 
same direction, but the air and gold much more than the air. Gold on glass in this 
medium, or gold in air, or glass in air, all gave results in the same direction, i. e. requii’ed 
direct rotation of the analyser to compensate for them. 
I proceeded to examine the other forms of gold; and first, the deposits on glass 
obtained by electric deflagration. These affected the ray of polarized hght exactly in 
the manner of gold-leaf, and that even at the distant parts of the deposit. It was most 
striking to contrast the thinnest and faintest portion of such a film uith the neighbour- 
ing parts of the glass from which it had been wiped off. It must be remembered that 
such a preparation is a layer of separate particles ; that these particles are not like those 
of starch or of crystals, for they have no action whilst in a plane perpendicular to the 
polarized ray ; nor have they a better action for being in a thick layer, as in the central 
parts of the deposit. The particles seem to form the equivalent of a continuous plate of 
transparent substance ; and as in such a plate it is the two surfaces which act, so there 
appears to be the equivalent of these two surfaces here ; which would seem to imply that 
the particles are so small and so near, that two or more can act at once upon the indi- 
vidual atoms of the vibrating ether. Their association is such as to present as it were 
an optical continuity. 
The gold films by phosphorus were then submitted to experiment, and gave exactly 
the same result. All of them depolarized, and requit’ed direct rotation of the analyser 
to arrive at a minimum, or to pass from the red to the blue tints. Graduated films, of 
which I should judge from the depth of tint that one place was at least twenty times as 
thick as another, gave the effect as well in the thinnest as the thicker or any inter- 
mediate part ; indicating that thickness of the plate, and therefore any quality equiva- 
lent to crystalline force of the particles, had nothing to do udth the matter. A glass 
beaker, which had been employed to contain ruby fluid, had a film of gold deposited on 
its inner surface so thin, as to be scarcely perceptible either by reflexion or othenAse. 
except by a ruby tint which appeared upon it in certain positions ; but being examined 
by a polarized ray, it gave an effect as strong and as perfect as gold-leaf, sho^ving how thin 
a film of gold was sufiicient for the purpose. This thin film appeared to be almost perfect 
in its continuity, for when the red image was brought in, direct rotation of the analyser 
reduced it to a minimum which was quite dark ; after Avhich, further rotation brought 
in a good blue image. The least touch of the finger removed the film of gold and all 
these effects with it. These films, though they are certainly porous to gas, and to water 
in some form, for it can evaporate from beneath them through its body, have erfdently 
optical continuity. 
In order to submit the gold fluids to experiments, cells were made of two glass plates. 
