Hypo does not keep well in solution, and should be made up in small quantities. It is better to keep the bottles containing it in a dark place, or to wrap them in paper. A small piece of chalk dropped in the solution will counteract or neutralize any trace of acid that may be formed.
A bottle which has contained hypo, or a dish in which it has been used for fixing purposes, should not be used for other chemicals. Hypo will penetrate glass or porcelain in a few days, and therefore contaminates any solution which is placed in a vessel which has once contained it.
The only printing process in which water is used as a clearing or fixing agent is in the cyanotype or "blue-print" process. This blue-print paper is coated with a solution of potassium ferrocyanide and ammonio-citrate of iron. Each of these salts is soluble in water, but when the paper with which they are coated is exposed to the action of light, the two substances combine and form a compound which is something of the nature of "Prussian blue." This compound is insoluble in water, and when a print is made on the sensitive paper and placed in water the salts which have not been acted upon by the light dissolve and wash away, while the compound formed by the union of the two salts under the light action remains, and the image is permanent.
A curious experiment may be made with the blue print. A paper is coated with an equal mixture of the two salts in equal proportions, and a blue print made on it in the usual way. When this print is dry, if it is placed in a solution of proto-nitrate of mercury the picture will soon disappear. If this apparently clear paper is washed and dried, and ironed with a hot iron—not hot enough to scorch the paper—the picture will gradually reappear, but the color will now be brown instead of blue. When this picture is placed for a few days in a book or portfolio it will fade away, but can be restored by again pressing with a hot iron.
The next paper will give the explanation of the toning or—as the early photographers termed it—the coloring process of the silver print.
THE SECOND SUMMER,
many mothers believe, is the most precarious in a child's life; generally it may be true, but you will find that mothers and physicians familiar with the value of the Gail Borden Eagle Brand Condensed Milk do not so regard it.—[Adv.]