It is well known that when thin leaves of gold or silver are mounted upon glass and heated to a temperature which is well below a red heat, a remarkable change of properties takes place, whereby the continuity of the metallic film is destroyed. The result is that white light is now freely transmitted, reflection is correspondingly diminished, while the electric resistivity is enormously increased. A simple method of illustrating this extraordinary change is to mount a sheet of silver leaf between two clean lantern plates, clip them lightly together by means of wire paper fasteners or other suitable means, and then heat gradually to a temperature of not more than 500°C. This can be done conveniently by placing the plates on a thin fire-brick in a cold gas muffle, and then raising the temperature to the desired point. The gas should now be turned off, and the glass plates allowed to cool slowly, so as to avoid cracking. They can then be bound with strips like an ordinary lantern plate, and a permanent example of transparent silver is obtained. It will be found that such a plate transmits the light of the electric lantern almost as readily as ordinary glass, and does not produce any change of colour. The great transparency of the film may be shown by placing the plate upon printing or writing, and photographing the characters through the plate. Every detail of the characters can be reproduced with remarkable clearness. At first sight it is perhaps difficult to conceive that so distinct an impression could be obtained through what was originally a perfectly opaque sheet of silver, and which has only been once heated to a moderate temperature.
Transparent silver and other metallic films
