(2) Photographic plates are coated with bromide of silver, a yellow substance, which may be prepared similarly to the previous precipitate by adding potassium bromide solution (instead of table salt) to the nitrate of silver. Its appearance does not change rapidly under the influence of light, but if first exposed and then treated with a developing solution the yellow color very soon changes to black—finely divided metallic silver being, in fact, produced. Actually, light more readily alters the constitution of the bromide than that of the white chloride, but the former knows better how to preserve an outward appearance of composure.
(3) Suppose, now, another solution be made, this time of the fixing salt known familiarly to every camera knight as “Hypo.” When this is added to either the white chloride or yellow bromide precipitates above noticed, they gradually dissolve away, except such portions as have changed color under the influence of light.
Such action constitutes the process of fixing a photograph, whereby the sensitive silver compound is removed from those parts of the paper or plate which have more or less escaped the influence of light.
(4) This experiment is an aquatic performance in which one actor only—our old acquaintance Hypo—takes part. Provided proper care be taken in the preparatory stages, it will afford at the climax as excellent a spectacle as many another more complex.
Fig. 5.—Preparing saturated solution.
Fig. 6.—Grooved cork for “dripping” solutions.
A tumbler glass full of saturated solution has first to be prepared, and this is best done by tying about 1⁄2 lb. of Hypo in a piece of muslin, so that it may be held against the rim of the glass and allowed to hang in hot water after the manner of [Fig. 5]. When an appreciable quantity of the salt has dissolved, the liquid being but lukewarm, the muslin bag may be removed and the solution stirred gently. Then it must be stood somewhere firm, and allowed to remain absolutely undisturbed until cold. There should then be a glass full of clear liquid, and the phenomenon is at hand.
Let the smallest crystal of solid Hypo be dropped in this liquid, or let it but be disturbed, and behold! a wonderful transformation proceeds, until the glass interior becomes a shimmering mass of sparkling crystals. The reason of this curious behavior is not far to seek. Hypo, in common with most chemicals, dissolves to a greater extent in hot water than in cold, but is different, inasmuch as the excess of salt does not settle out as the solution becomes cold. Cold solution is therefore really over-saturated, and to such an extent that only an extra crystal or the least disturbance is sufficient to upset the delicate balance, upon which climax the great excess of Hypo soon settles out.