444 Professor Faraday, on the [June 12, 



WEEKLY EVENING MEETING, 



Friday, June 12. 



Sir Benjamin Coulins Brodie, Bart. D.C.L. F.R.S. Vice- 

 President, in the Chair. 



Professor Faraday, D.C.L. F.R.S. 

 On the Helations of Gold to Light. 



This subject was brought forward on the 13th of June of last year, 

 and in the account of that evening, at page 310, vol. ii. of the Pro- 

 ceedings of the Royal Institution, will be found a description of 

 some of the proofs and effects then referred to and illustrated ; 

 the following additional remarks will complete the account up to 

 this time. The general relations of gold leaf to light were de- 

 scribed in the former report. Since then, pure gold leaf has been 

 obtained through the kindness of Mr. Smirke, and the former ob- 

 servations verified. This was the more important in regard to the 

 effect of heat in taking away the green colour of the transmitted 

 light, and destroying to a large extent the power of reflexion. The 

 temperature of boiling oil, if continued long enough, is sufficient for 

 this effect ; but a higher temperature (far short of fusion) pro- 

 duces it more rapidly. Whether it is the result of a mere breaking 

 up by retraction of a corrugated film, or an allotropic change, is 

 uncertain. Pressure restores the green colour ; but it also has the 

 like effect upon films obtained by other processes than beating. 

 Corresponding results are produced with other metals. 



As before stated, films of gold may be obtained on a weak 

 solution of the metal, by bringing an atmosphere containing vapours 

 of phosphorus into contact with it. They are produced also when 

 small particles of phosphorus are placed floating on such a solu- 

 tion ; and then, as a film differing in thickness is formed, the 

 concentric rings due to Newton's thin plates are produced. These 

 films transmit light of various colours. When heated they become 

 amethystine or ruby ; and then when pressed, become green, just 

 as heated gold leaf. This effect of pressure is characteristic of 

 metallic gold, whether it is in leaf, or film, or dust. 



Gold wire, separated into very fine particles by the electric 

 deflagration, produces a deposit on glass, which, being examined, 

 either chemically or physically, proves to be pure metallic gold. 



