6 The Ethereal Hypothesis of Light. [Jan., 



by transmitted light is green ; by incident light, yellow, and of a 

 metallic lustre. 



" When gold-leaf is laid upon glass, and its temperature raised 

 considerably without disturbance, either by the blow-pipe or an 

 ordinary argand-burner, it seems to disappear, i. e. the lustre passes 

 away, the light transmitted is abundant and nearly white;"* but 

 " when gold, rendered colourless by annealing, is subjected to pres- 

 sure, it again becomes a green colour," .... and " the green colom- 

 can be again taken away by heat to appear again by renewed pres- 

 sure."! Again, gold in a minutely di-vided condition caused by 

 deflagration, transmitted violet, green, or ruby rays; but by re- 

 flected Hght " it is golden and metaUic."J " It is evident that all 

 the colour's described are j)roduced by one and the same substance, 

 namely, gold, the only apparent diflerence being the state of division 

 and different degrees of the application of heat;"§ . . . "and I think 

 I am justified by my experiments in stating that fine gold particles 

 so loosely deposited that they wipe ofi" by a hght touch of the 

 finger, and possessing one conjoint structure, can in one state 

 traDsmit hght of a blue-grey colour, or can by heat be made to 

 transmit hght of a ruby colour, or can by pressure from either of 

 these former states transmit light of a green colour, all these modi- 

 fications being due to gold as gold."|l That it is the disposition of 

 the particles which causes the modifications of colour is further 

 shown by the author, when he saysH that thin films of gold pre- 

 pared by phosphorus give "a feeble grey- violet" by transmitted 

 light ; if the films are a little thicker they give " a violet ;" but 

 " superposition of several grey -violet films does not produce a green 

 tint, but only a diminution of light without change of colour." 

 Yet it wiU be remembered that a sheet of gold-leaf gives a bright 

 green. 



Another result of Faraday's observations is that vapours and 

 gases will pass through these films ;** and their appearance with a 

 power of 700 hnear is reported to be "slightly granular."tf 



* ' Phil. Trans., 1857,' p. 148. 



t Ibid., p. 141). It may be as well to mention here what Faraday thoup:ht in 

 relation to the cause of the cLange. At p. 141) he says: — " As to tlie essential cause 

 of this change of colour, more investigation is required to decide what tliat may bo. 

 As already mentioned, it miglit be thouglit that the gold-leaf had run up into 

 separate particles. . . . On tlie Avhole I incline to this opinion." Let me add in 

 reference to these "remarks of Faraday, that on examining with a microscope some 

 " gold bronze," whicli I know consisted of very fine particles of gold, I found that 

 by transmitted light tlieygave in the massa yellowish-rrd colour, and each particle 

 precisely resembled minute flakes of crumpled gold-leaf; but wlien I subjected the 

 dust to pressure between two sides, not alone each particle, but the whole aggre- 

 gation of them assumed the characteristic green hue — the change being, however, 

 more ajiparent in the individuid particles. 



X Ibid., p. 152. § Ibid., p. 153. || P. 153. ^ Pp. 155-6. 



** " Experimentally, also, I find that vapoui-s and gases can pass through 

 them," p. 150. 



ft P. 157 



