404 Prof. Faraday on the Experimental Relations 
edge of the leaf, it will pass between the glass and gold, and the 
latter will be perfectly stretched ; if the water be then drained 
out, the gold-leaf will be left well extended, smooth, and ad- 
hering 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 cyanide 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 varnish, the gold film will be left cemented to the glass*. 
In this manner the leaf may be obtained so thin, that I think 
50 or even 100 might be included in a single progressive undu- 
lation of light. But the character of the effect on light is not 
changed, the light transmitted is green,as before; and though that 
ereen tint is due toa 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 irregular 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 pro- 
portion of light which passes through may be accepted as an in- 
dication. Yet this irregular plate does not cause any sensible 
distortion of an object seen through it, that object being the line 
of light reflected from a fine wire in the focus of a moderate 
microscope. Nor perhaps was any distortion due to consecutive 
convexities and concavities to be expected; for when the thicker 
parts of the leaf were examined, they seemed to be accumulated 
plications of the gold, the leaf appearing as a most irregular and 
crumpled object, with dark veins running across both the thicker 
and thinner parts, and from one to the other. Yet in the best 
microscope, and with the highest power, the leaf seemed to be 
continuous, the occurrence of the smallest sensible hole making 
that continuity at other parts apparent, and every part possess- 
ing its proper green colour. How such a film can act as a plate 
* Air-voltaic circles are formed in 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 surface, both dissolved ; but 
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 were obtained; from sixty to a hundred- 
fold 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 forma- 
tion of the films to be described hereafter. 
