The presence of the golden-colored zone in the place mentioned is worthy of remark, as in the tables of Newton of the colors 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 attacked in this manner. Chlorine and bromine on silver; oxygen on steel; chlorine and bromine on titanium, bismuth, &c., commence their colors in the same way. Copper, however, 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 color of the copper itself. With this single particularity, this metal undergoes the same alterations as the others.
The action of light on the different colors of the iodide of silver is very interesting: the most correct way of studying this is to protect one half of a system of colored 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 colors appear to be similarly affected by the action of light, though to a very slight degree, acquiring a trifling accession in their brilliancy. It has already been remarked that light destroys the adherence of the external invisible film; the same thing obtains with the second or gold-colored film, which turns green, _but only to a certain depth_ of the film, as may be proved by slightly rubbing the part thus altered; the green color is then seen to disappear, and beneath the pulverulent portion thus removed is found the gold color, having almost the same appearance as before the plate had been exposed. As this experiment may be repeated several times with the same results, it shows to how inconceivably small a depth the light has acted to produce this effect.
To ascertain what would take place on augmenting the thickness of the portion turned green, and the adherence of which was destroy ed, a piece of iodine was placed on the plate so that its vapor, by expanding, might arrive upon the green, at the same time the whole being kept from the light; the result was that the additional film combined with the one already existing, producing a blue, being the color which would have resulted by the combination of the unaltered yellow films. I have found no chemical substance possessing the power of arresting, or in any way influencing these changes of color; strong adds, provided they do not attack the silver--for then, of course, the experiment would be destroyed,--and alkalies in concentrated solution, allow the action of light to go on as usual. The hyposulphate of soda, and ammonia in solution have no longer the power of dissolving the green film as they had before the action of light.