Art. LXI.—On certain Results obtained upon some of the Argentiferous Salts which are affected by Light.
[Read before the Wellington Philosophical Society, 11th October, 1879.]
It may be known, although I can get no direct intimation of this, that iodide of silver, like the chloride, requires the presence of water or its constituents ere photographic effects can be induced upon it; but, whether or no, a description of the results which I have obtained in connection with this point will be, I think, of some interest to you, and I therefore give one, and attach thereto a statement of other results obtained with these substances bearing upon the mode in which this photographic effect is produced:—
Iodide of silver is not photographically affected even by direct sunlight, if it is kept the while at a temperature approaching to 100° C.; the presence of water or aqueous vapour here does not affect the result.
At a temperature of 100° C., and in presence of water, chloride of silver, on the other hand, darkens—though but slowly—in direct sunlight. In aqueous vapour, at about 160° C., it does not darken.
A strong aqueous solution of sodic-chloride in contact with these silver salts renders them impervious to the action of light, even at common temperatures.
At a temperature approaching 100° C. photographic effects produced upon argentic-iodide are obliterated.
A considerable quantity of pure argentic-iodide, after being stirred in the light till it became of a light colour, did not either lose or gain in weight thereby,—that is, appreciably to me. At 100° C. its colour became a full yellow, relapsing to a pale tint when cooled to a common temperature.
The product of sunlight upon even pure argentic-chloride varies both in colour and composition according as to whether this compound is alkalized or not. Thus, in a weak solution of potash, the product is of a very dark brown or black colour, and this is permanent under the circumstances. In weak acetic or hydrochloric acid, it evolves gas (probably oxygen), and acquires a pale red colour. A large volume of distilled water
added thereto produces the same kind of change. This may be an oxy-chloride. If, on the other hand, an alkali is omitted, the silver salt acquires a pale-reddish colour, and undergoes that chemical change which is generally imputed to it when subjected to light,—that is, it passes to argentous-chloride with evolution of chlorine; while hydrochloric acid is also produced in a secondary way (no doubt from the chlorine), and this, being free, is antagonistic to the formation of that dark product which results, as I have said, when photographic effect is produced in the presence of alkaline matter.
Iodide of silver forms, as I have before shown,* a bright yellow compound with mercuric-iodide, as administered thereto in the form of mercuro-iodide of potassium. This compound, I have just ascertained, contains only about one-quarter per cent. of mercuric-iodide.
The quantity of mercuric-chloride which has to be present in argentic-chloride so as to render it insensitive to light, need not be more than 1·16 per cent. of the whole compound.
Most of the aniline dyes are absorbed to a small extent by these silver salts.
Argentic-chloride is to a very slight extent hygroscopic. In the dark at 22° C. a well washed sample of this compound gained ·11 per cent. of water.
Oxidizing agents generally, when applied to argentic-iodide which has been faded by light, turn its colour to a full yellow.
Argentic-iodide precipitated from an excess of argentic-nitrate has its dull yellow colour changed to bright yellow by ammonia; but this yellow relapses to the normal hue upon the addition of water thereto in quantity. But if the argentic-iodide prepared in this manner is first thoroughly modified by light, ammonia then browns in the place of yellowing it, and the addition of water causes a relapse to its former colour.
I will for the present only make the following few remarks upon these notes.
In 1 and 2 is shown in a marked manner the necessity of water in some form for the production of photographic effect upon argentic-iodide, and also of a temperature very far below its fusing point, facts which should, when fully realized by chemists, lead to a knowledge of the exact molecular change which such an effect involves.
The opposing effect of sodic-chloride to the action of light on silver salts (3), when taken in connection with results 8, 9, and 10, seems due to the formation of a double salt therewith which is not sensitive to this agent.
[Footnote] * Trans. N. Z. Institute, Vol. IX., p. 555.
The effect of a gentle heat in obliterating photographic impressions (4) is no doubt called into play, where, as Claudet observes, “exposure to the red rays neutralizes the effect previously produced on a sensitive surface by white light.”
From results Nos. 7, 8, 9, and 10, it is seen that these silver salts have a predisposition to combine with small or even minute quantities of certain substances, the same as molybdic acid and its congeners have in relation to phosphoric acid, and this, without doubt, indicates a molecular change throughout, and one which, at least in the case of argentic-iodide, is possibly of the same nature in respect to structural form as that which light induces. It has been ascertained by Vogel that in the case of this salt (argentic-iodide) no silver or iodine is liberated by light—the change is molecular only; so that the experimental results detailed above may have a significance in regard to the mode in which light produces this. But being a change apparently of this character only, it is not a little singular that water, or its constituents, is necessary for its production; and it was this that led me to make experiment (5), under the idea that water is assimilated by this salt when photographic effect takes place upon it. The result, however, as is seen, does not show this to be the case; but further experiments seem necessary here.
From the results last described (11, 12) it is seen that most oxidizing substances have the property of putting argentic-iodide, altered by light, back to its normal state, and that this salt comports itself with ammonia according as to whether light has been permitted to exert its influence thereupon or not, results which have no doubt an intimate relation to the phenomena under consideration, but in what way it is difficult as yet to discover. The primary effect of ammonia, however, in the last result is, it would seem, merely to form feebly ammoniacal compounds with the silver salts, which are so unstable as to be decomposable by water.