4.06 Prof. Faraday on the Experimental Relations 
vided 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 division. 
When the gold-leaf is laid by water on plates of rock-crystal, 
and then gradually heated in a muffle not higher than is neces- 
sary, an excellent result is obtamed. The gold is then of a uni- 
form pale brown colour by common observation, but when 
examined by a lens and an oblique light, all the mottle of the 
original leat appears. It adheres but very slightly to the rock- 
erystal, and yet can bear the application of the pressure now to 
be described. 
When gold rendered colourless by annealing is subjected to 
pressure, it again becomes of a green colour. I find a convex 
surface of agate or rock-crystal having a radius of from a quar- 
ter 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 in- 
ereased. When the gold-leaf has been heated on glass in a 
muffle, it generally adheres so well as to bear streaking with 
the convex rock-crystal, and then the production of the reflect- 
ing surface and the green transmission is very striking. In 
other forms of gold film, to be described hereafter, the green- 
ing effect of pressure (which is general to gold) is still more 
strikingly manifested, and can be produced with 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 inyes- 
tigation is required to decide what that may be. As already 
mentioned, it might be thought that the gold-leaf had run up 
into separate particles. If it were so, the.change of colour b 
division is not the less remarkable, and the case would fall into 
those brought together under the head of gold fluids. On the 
whole, I incline to this opinion; but the appearance in the mi- 
croscope, 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, throw 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 
