OF GOLD (AND OTHEE METALS) TO LIGHT. 
147 
and an arrangement of sulphate of lime plates, it was found that other rays than the 
green could be transmitted by the gold-leaf. The yellow rays appeared to be those 
which were first stopped or thrown back. Latterly I have obtained some pure gold-leaf 
beaten by Maeshall, of which 2000 leaves weighed 408 grains, or 0'2 of a grain per 
leaf: its reflected colour is orange-yellow, and its transmitted colour a warm green. 
Gold alloy containing 25 per cent, of silver produces pale gold-leaf, which transmits a 
blue purple hght, and extinguishes much more than the ordinary gold-leaf. 
So a leaf of beaten gold occupies in average thickness no more than from ^th to |^th 
part of a single wave of hght. By chemical means, the film may be attenuated to such 
a degree as to transmit a ray so luminous as to approach to white, and that in parts 
which have every appearance of being continuous in the microscope, when viewed with 
a power of 700. For this purpose it may be laid upon a solution of chlorine, or, better 
still, of the cyanide of potassium*. If a clean plate of glass be breathed upon and then 
brought carefully upon a leaf of gold, the latter will adhere to it ; if distilled water be 
immediately apphed at the edge of the leaf, it will pass between the glass and gold, and 
the latter ^vill be perfectly stretched ; if the water be then drained out, the gold-leaf will 
be left well extended, smooth, and adhering to the glass. If, after the water is poured 
off, a weak solution of cyanide be introduced beneath the gold, the latter will gradually 
become thinner and thinner ; but at any moment the process may be stopped, the cya- 
nide washed away by water, and the attenuated gold film left on the glass. If towards 
the end a washing be made with alcohol, and then with alcohol containing a little var- 
nish, the gold film will be left cemented to the glass f. 
In this manner the leaf may be obtained so thin, that I think 60 or even 100 might 
be included in a smgle progressive undulation of light. But the character of the effect 
on hght is not changed, the light transmitted is green, as before ; and though that green 
tint is due to a condition of the gold induced by pressure, it as yet remains unchanged 
through all these varieties of thickness and of proportion to the progressive or the lateral 
undulation. 
Gold-leaf, either fine or common, examined in the microscope, appears as a most irre- 
gular thing. It is everywhere closely mottled or striated, according as a part at the 
middle or the edge of a leaf is selected, minute portions which are close to, other parts 
being four or five times as thick as the latter, if the proportion of light which passes 
through may be accepted as an indication. Yet this irregular plate does not cause any 
* The chlorine leaves a film of chloride of silver behind, the cyanide leaves only metal. 
t Air-voltaic circles are formed ia these cases, and the gold is dissolved almost exclusively under their 
influence. When one piece of gold-leaf was placed on the surface of a solution of cyanide of potassium, and 
another, moistened on both sides, was placed under the sm-face, both dissolved ; hut twelve minutes sufficed 
for the solution of the first, whilst above twelve hours were required for the submerged piece. In weaker 
solutions, and with silver also, the same results Avere obtained ; from sixty to a hundredfold as much time 
being required for the disappearance of the submerged metal as for that which, floating, was in contact both 
with the air and the solvent. An action of this kind has probably much to do with the formation of the films 
to be described hereafter. 
X 2 
