1857.] of Gold (and other Metals] to Light. 397 



and then gradually heated in a muffle not higher than is neces- 

 sary, 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 light, 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 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 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 transmitted 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 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 in- 

 vestigation 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 by 

 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 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, throw 

 doubts in the way of such a conclusion. 



