149 
OF GOLD (AFfD OTHEE METALS) TO LIGHT. 
occurs ; the surrounding medium also may change, and be air, oil or carbonic acid, with- 
out causing alteration. Nor has the gold disappeared ; a piece of leaf, altered in one part 
and not in another, was divided into four equal parts, and the gold on each converted 
by chlorine gas into crystallized chloride of gold ; the same amount was found in each 
di\ision. 
When the gold-leaf is laid by water on plates of rock-crystal, and then gradually 
heated in a muffle not higher than is necessary, an excellent result is obtained. The 
gold is then of a uniform pale brown colour by common observation, but when examined 
by a lens and an oblique hght, all the mottle of the original leaf appears. It adheres 
but very slightly to the rock-crystal, and yet can bear the application of the pressure 
now to be described. 
"When gold rendered colourless by annealmg is subjected to pressure, it again becomes 
of a green colour. I find a convex sui’face of agate or rock-crystal having a radius of 
from a quarter to half an inch very good for this purpose, the metal having very little 
tendency to adhere to this substance. The greening is necessarily very imperfect, and 
if examined by a lens it will be evident that the thinner parts of the film are rarely 
reached by the pressure, it being taken off by the thicker corrugations ; but when reached 
they acquire a good green colour, and the effect is abundantly shown in the thicker parts. 
At the same time that the green colour is thus reproduced, the quantity of light trans- 
mitted is diminished and the quantity of light reflected is increased. When the gold- 
leaf has been heated on glass in a muffle, it generally adheres so well as to bear streak- 
ing with the convex rock-crystal, and then the production of the reflecting surface and 
the green transmission is very striking. In other forms of gold film, to be described 
hereafter, the greening effect of pressure (which is general to gold) is still more strikingly 
manifested, and can be produced mth the touch of a card or a finger. In these cases, 
and even with gold-leaf, the green colour reproduced can be again taken away by heat, 
to appear again by renewed pressure. 
As to the essential cause of this change of colour, more mvestigation is required to 
decide what that may be. As afready mentioned, it might be thought that the gold-leaf 
had run up into separate particles. If it were so, the change of colour by division is not 
the less remarkable, and the case w'ould fall into those brought together under the head 
of gold fluids. On the whole, I incline to this opinion ; but the appearance in the micro- 
scope, the occurrence of thin films of gold acting altogether like plates and yet not 
transmitting a green ray until they are pressed, and their action on a polarized ray of 
light, tlirow doubts in the way of such a conclusion. 
It may be thought that the beating has conferred a uniform strained condition upon 
the gold, a difference in quality in one direction which annealing takes away ; but when 
the gold is examined by polarized light, there is no evidence as yet of such a condition, 
for the green and the colourless gold present like results ; and there is a little difficulty 
in admitting that such an frregular corrugated film as gold-leaf appears to be can pos- 
sess any general compression in one direction only, especially when it is considered 
