528 Prof. Faraday on the Experimental Relations 
they still act with chemical agents as gold. Pressure then con- 
fers the green colour, which heat takes away, and pressure re- 
confers, All these changes occur with particles attached to the 
substances which support them by the slightest possible mecha- 
nical force, just enough indeed to prevent their coalescence and 
to keep them apart and in place, and yet offering no resistance 
to any chemical action of test agents, as the acids, &e., not 
allowing any supposition of chemical action between them and 
the body supporting them. Still this gold, unexceptionable as 
to metallic state, presents different colours when viewed by trans- 
mitted light. Ruby, green, violet, blue, &c. occur, and the mere 
degree of division appears to be the determining cause of many 
of these colours. The deflagrations by the voltaic battery lead 
to the same conclusion. 
The gold films produced by phosphorus have every character 
belonging to the metallic state. When thick, they are in co- 
lour, lustre, weight, &c. equal to gold-leaf, but in the unpressed 
state, their transmitted colour is generally gray, or violet-gray. 
The progression of their lustre and colour is gradual from the 
thickest to the thinnest, and the same is generally true, if thick 
films are gradually thinned and dissolved whilst floating on 
solvents ; the thick and the thin films must both be accepted as 
having the same amount of evidence for their metallic nature. 
When subjected to chemical agents, both the thick and the thin 
films have the same relations as pure metallic gold. These re- 
lations are not changed by the action of heat, yet heat shows 
the same peculiar effect that it had with preparations of gold 
obtained by beating, or by electric deflagrations. The remark- 
able and characteristic effect of pressure is here reproduced, and 
sometimes with extraordinary results; since from the favourable 
manner in which the particles are occasionally divided and then 
held in place on the glass, the mere touch of a finger or card is 
enough to produce the result. Yet with gold thus proved to be 
metallic, colours including gray, gray-violet, green, purple, ruby, 
especially by heat, and green again by pressure, and by thinning 
of gray films, may be obtained by transmitted light, almost all 
of them at pleasure. 
It may be thought that the fluid preparations present more 
difficulty to the admission, that they are simply cases of pure 
gold in a divided state ; yet I have come to that conclusion, and 
believe that the differently-coloured fluids and particles are quite 
analogous to those that occur in the deflagrations and the films. 
In the first place they are produced as the films are, except that 
the particles are separated under the surface and out of the con- 
tact of the air ; still, when produced in sufficient quantity against 
the side of the containing vessel to form an adhering film, that 
