62 LECTURES TO SCIENCE TEACHERS. 



alternations you have also alternations in the character of 

 reflected light ; so that you may say the substance is alter- 

 nately opaque and transparent, comparatively speaking only, 

 as regards the transmitted light, and, corresponding to these 

 alternations, it behaves as regards reflection alternately as a 

 metal and as a vitreous substance. That shows how the 

 coloured reflection, where it does exist it is a phenomenon, 

 comparatively speaking, rare is connected with the quasi- 

 metallic opacity of the substance as regards transmission. 



You may say that if that be the case the colour of gold 

 ought to be not yellow at all by transmission ; nor is it. Gold 

 leaf is thin enough to allow some light to pass through it 

 otherwise than by mere holes, which occur accidentally here 

 and there, and that transmitted light is green. I have here a 

 little chloride of gold in solution. I put a little protosul- 

 phate of iron in it, and if the experiment is properly per- 

 formed you obtain what is not really a solution of gold, but 

 gold suspended in a state of exceedingly fine division ; and 

 in that way, when the fluid is looked through, you get it 

 distinctly blue, which is the real transmission colour of gold. 

 I have seen the same thing with regard to copper. Dr. Percy 

 gave me a specimen of a very curious glass, which I intended 

 to have brought with me. The ordinary red glasses are 

 coloured by suboxide of copper, which is put over a piece of 

 colourless glass in a film of copper-salt so thin that you do not 

 see any colour at all by light transmitted directly across, 

 but where you look through obliquely you can just see the 

 faintest possible blueness. The film of copper-salt is reduced 

 by a suitable agent to a silicate of suboxide, which gives 

 that beautiful red colour, which is contained in a film thinner 

 than the thinnest paper. In this case the glass was covered 

 with copper in a similar manner, but it was a deep blue 

 by transmitted light, and if you play on any particular spot 

 with a blow-pipe it becomes sensibly colourless. The 

 colouring matter was copper, but in what state 1 Evidently 

 in this case the reduction necessary for reducing the oxide 

 of copper to suboxide had gone on rather too far, the 

 copper was reduced to the metallic state ; you looked through 

 the copper, and it was seen to be blue. So that you see 

 that in the same sense in which the coat of an English 

 soldier is red, the colour of gold is blue or green, and the 

 colour of copper is blue. There is the same relation there 



