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but by far the greater part are so minute as not to be distinguishable 

 separately. The general film is of different colours by transmitted 

 light, being grey, violet, or green; and often on the central or 

 nearest part of the discharge, where the heat has been active, is of a 

 fine ruby colour. All these particles act with acid and chemical 

 reagents as gold acts ; and there is no reason to believe they are 

 anything other than metallic gold. They appear with precisely the 

 same colours and characters, whether the deflagrations are made in 

 common air, in oxygen, or in hydrogen ; and whether the deposits 

 are formed on glass, rock-crystal, topaz, or mica. When heated by 

 any ordinary means, the green and grey parts change to a ruby or 

 ruby-amethystine colour, and that whether surrounded by air, or 

 vapour of alcohol or ether. Even after heating, they adhere only as 

 a dust to the plates, except when the temperature applied to those on 

 glass has been very high. Agate pressure confers the green character 

 on the heated deposits, and also, in frequent cases, upon that which 

 has not been heated. All things considered, there can be no reason 

 to doubt that the deposits thus made to vary in the colour of the 

 transmitted light, consist of pure metallic gold. 



Thin films of Gold, If a very weak solution of chloride of gold, 

 free from excess of acid, and containing about 1^ grain of metal to 

 2 or 3 pints of water, be placed in a very clean glass or glazed vessel, 

 in a quiet place, and then two or three small particles of phosphorus 

 be laid floating on the surface, and the whole covered over and left 

 for twelve or more hours, the gold will be reduced, covering the 

 whole of the surface with a film, thicker near the phosphorus than 

 at other parts. This film may be raised from the fluid by plates of 

 glass, and washed and dried on the plates, and is then ready for ex- 

 amination. The thinner parts of such a film are scarcely visible, 

 either by reflected or transmitted light ; the transition to thicker 

 parts is gradual, the thickest being opaque, and their reflexion that 

 of dense gold. The colour by transmitted light varies, being grey, 

 green, or dull violet. The films are porous, and act as pure gold, 

 resisting all the agents which metallic gold resists. When heated, 

 the transmitted colour changes towards amethyst and ruby ; and 

 then the effect of pressure in producing a green colour is in many 

 cases very remarkable, even a touch with a card or the finger being 

 able to cause the change. 



