157 
or GOLD (AJ^D OTHEE METALS) TO LIGHT. 
from grey-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 boihng oil, if continued, being sufiicient; but on applying 
pressure at the same spot, the power of transmitting green light was restored to the par- 
ticles. 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 no such evident appearance in these cases, 
as in the unattached gold-leaf of the particles running up into globules. 
A given film, examined very carefuUy in the microscope by transmitted lamp-light, 
with an aperture of 90° and power of 700 linear, presented the following appearances. 
The unheated part was of a grey colour, and by careful observation was seen to be 
slightly granular. By very close observation this grey part was often resolvable into a 
mixture of green and amethystine striae, it being the compound effect of these which in 
general produce the grey sensation in the eye. Wlien a part of such a film was heated, 
the transmitted colour was changed from grey to purple, as before described, and the 
part thus heated was evidently more granular than before. This difference was con- 
firmed in other cases. That the heated part should thus run up, seems to show that 
many of the particles must have been touching though they did not form a continuous 
film ; and on the other hand, the difference between the effect here and with unattached 
gold-leaf, shows that the degree of continuity as a film must be very small. When 
these heated films were greened by agate pressure, or the drawing pressure of a card, the 
green parts remained granulated, apparently in the same degree as when purple. The 
green was not subjective or an effect of interference, but a positive colour belonging to 
the gold in that condition. Every touch of the agate was beautifully distinct as a written 
mark. The parts thus greened and the purple parts appeared to transmit about the 
same amount of light. Though the film appeared granulated, no impression was made 
upon the mind that the individual particles of which the film consisted were in any 
degree rendered sensible to the eye. 
The unheated gold films when pressed by agate often indicated an improved reflective 
power, and the light transmitted was also modified ; generally it was less, and occasion- 
ally tended towards a green tint ; but the effect of pressure was by no means so evident 
as in particles which had been heated. 
Films of some other metals were reduced by phosphorus in like manner, the results in 
all these cases being of course much affected by the strength of the solution and the 
time of action ; they are briefly as follows. Palladium : a weak solution of the chloride 
