coloured Films formed by Iodi?ie, fyc. upon Metals. 429 



loured circles, until these come in contact at their circum- 

 ferences, when the two systems will slowly coalesce and pro- 

 duce one common outline of the form of an ellipsis. 



As the colours formed on various metals by the above-men- 

 tioned agents are very similar to one another, it may be suffi- 

 cient to examine in particular those produced on silver by 

 iodine. 



The external film of the iodide of silver rings, which cor- 

 responds to the central black spot in those of Newton, is com- 

 pletely invisible, it being impossible to perceive any difference 

 between the parts so covered, and those where the metal is 

 intact ; but by exposing half the plate to the influence of light, 

 whilst the other part remains covered, the silver is then found 

 darkened far beyond the limits of the external gold-coloured 

 zone, where previously the surface was perfectly clear. The 

 dark film thus rendered apparent is now liable to be rubbed 

 off by the slightest friction, whereas before it was very adhe- 

 rent to the subjacent surface. The first zone is of a pale gold 

 colour, which assumes a deeper tint as the thickness of the 

 film increases : the second zone is blue, the third white ? 

 after these appear the different colours of the spectrum in re- 

 gular succession, as in the films studied by Newton and others, 

 viz. yellow, orange, red, blue, green, yellow, &c. The pre- 

 sence of the golden-coloured zone in the place mentioned is 

 worthy of remark, as in the tables of Newton of the colours 

 presented by films of various thicknesses, the blue is stated as 

 immediately following the black. The same gold film is the 

 first which appears on most metals when their surface is at- 

 tacked in this manner. Chlorine and bromine on silver ; 

 oxygen on steel ; chlorine and bromine on titanium, bismuth, 

 &c, commence their colours in the same way. Copper, how- 

 ever, is in one respect an exception, this metal first becoming 

 of a dark red, which increases to a ruddy brown and then 

 changes into blue ; this deviation is fully accounted for by the 

 colour of the copper itself; with this single particularity, this 

 metal undergoes the same alterations as the others. 



The action of light on the different colours of the iodide of 

 silver is very interesting : the most correct way of studying 

 this is to protect one half of a system of coloured rings by an 

 opake screen, while the other half is exposed for a short time 

 to the influence of the solar rays. The golden zone undergoes 

 the greatest change ; at first it grows darker, then red, and 

 at length is converted into a beautiful green. The blue film, 

 which comes next in thickness, suffers considerable alteration 

 in its tint, assuming a much deeper and more brilliant shade ; 

 the rest of the colours appear to be similarly affected by the 



