of Gold and other Metals to Light. “405 
on polarized light in the manner it does, is one of the queries 
suggested by the phenomena which requires solution. 
When gold-leaf is laid upon glass and its temperature raised 
considerably without disturbance, either by the blowpipe or an 
ordinary Argand gas-burner, it seems to disappear, 7%. e. the 
lustre passes away, the light transmitted is abundant and nearly 
white, and the place appears of a pale brown colour. One would 
think that much of the metal was dissipated, but all is there, 
and if the heat has been very high (which is not necessary for 
the best results), the microscope shows it in minute globular 
portions. A comparatively low heat, however, and one unable 
to cause separation of the particles, is known to alter the mole- 
cular condition of gold, and the gold-beater finds important ad- 
vantage in the annealing effect of a temperature that does not 
hurt the skins or leaves between which he beats the metal. 
It might be supposed that the annealed metal, in contracting 
from the constrained and attenuated state produced by beating, 
drew up, leaving spaces through which white light could pass, 
and becoming itself almost insensible through the smallness of 
its quantity ; and if gold-leaf unattached to glass be heated care- 
fully with oil in a tube, it dces shrink up considerably even 
before it loses its green colour, which finally happens. But if 
the gold-leaf laid upon glass plates by water only be carefully 
dried, then introduced into a bath of oil and raised to a tempe- 
rature as high as the oil can bear for five or six hours, and then 
suffered to cool, the plates, when taken out and washed, first in 
camphine and then in alcohol, present specimens of gold which 
has lost its green colour, transmits far more light than before, 
and reflects less, whilst yet the film remains in form and other 
conditions apparently quite unchanged. Being now examined 
in the microscope, it presents exactly the forms and appearance 
of the original leaf, except in colour; the same irregularities ap- 
pear, the same continuity, and if the destruction of the green 
colour has not been complete, it will be seen that it is the 
thicker folds and parts of the mottled mass that retain the 
original state longest. 
This change does not depend upon the substance in contact 
with which the gold is heated*. If the leaf be laid upon mica, 
rock-crystal, silver or platinum, the same result occurs; the 
surrounding medium also may change, and be air, oil or carbonic 
acid, without causing alteration. Nor has the gold disappeared ; 
a piece of leaf, altered in one part and not in another, was di- 
* The disappearance of gold-leaf as metal, when mingled with lime, 
alumina and other bodies, and then heated, has been already observed ; 
and referred to oxidation (J. A. Buchner). See Gmelin’s ‘Chemistry,’ vi. 
p- 206, “ Purple oxide of gold.” 
