of Gold and other Metals to Light. 415 
Though the particles of these films adhere together strongly, 
as may be seen by their stiffness on water, still the films cannot 
be considered as continuous. If they were, those made by 
vapour of phosphorus could not thicken during their formation, 
neither could they dry on glass in the short time found sufficient 
for that purpose. Experimentally also, I find that vapours and 
gases can pass through them. Very thin films without folds 
did not sensibly conduct the electricity of a single pair of 
Grove’s plates ; thicker films did conduct ; yet with these proofs 
that these films could not be considered as continuous, they 
acted as thin plates upon light, producing the concentric rings 
of colours round the phosphorus at their first formation, though 
their thickness then could scarcely be the -,dth, perhaps not the 
soodth of a wave undulation of light. Platinum, palladium, and 
rhodium produced films, showing these concentric rings very well. 
Many of these films of gold, both thick and thin, which 
being of a gray colour originally, were laid on a solution of cya- 
nide of potassium to dissolve slowly, changed colour as they dis- 
solved and became green; if change occurred, it was always 
towards green. On the other hand, when laid on a solution of 
chlorine, the change during solution was towards an amethyst 
or ruby tint. The films were not acted upon by pure nitric, or 
hydrochloric, or sulphuric acids, or solutions of potassa or brine. 
They dissolved in damp chlorine gas, not changing in colour 
during the solution. I believe them to consist of pure gold. 
When these gold films were heated to dull redness they 
changed. The reflexion, though not much altered, was a little 
more metallic and golden than before; more light was trans- 
mitted after the heating and the colour had altered from green- 
ish to violet, or from gray-green to ruby or amethyst; and now 
two or three films superposed often gave a very ruby colour. 
This action is like that of heat on the particles separated by 
electric explosions. If not overheated, the particles were not 
fused to the glass, but could be easily wiped off. Whenever 
these heated particles were pressed by the convex agate, they 
changed in character and transmitted green light. Heat took 
away this character of the gold, the heat of boiling oil, if con- 
tinued, being sufficient; but on applying pressure at the same 
spot, the power of transmitting green light was restored to the 
particles. In many cases where the gold adhered sufficiently to 
the glass to bear a light drawing touch from the finger or a card, 
such touch altered the light transmitted from amethystine to 
green ; so small is the pressure required when the particles are 
most favourably disposed. 
Heating injured the conducting power for electricity of these 
films, no doubt by retraction of the particles, though there was 
