The hyposulphite of soda commonly employed by photographers is a neutral combination of hyposulphurous acid and the alkali soda. It is selected as being more economical in preparation than any other hyposulphite adapted for fixing. Hyposulphite of soda occurs in the form of large translucent groups of crystals, which include five atoms of water. These crystals are soluble in water almost to any extent, the solution being attended with the production of cold; they have a nauseous and bitter taste. Hyposulphite of Gold. (_See_ Gold, Hyposulphite of.) Hyposulphite of Silver. (_See_ Silver, Hyposulphite of.) Iceland Moss. _Cetraria Islandica._--A species of lichen found in Iceland and the mountainous parts of Europe; when boiled in water, it first swells up, and then yields a substance which gelatinizes on cooling. It contains lichen starch; a bitter principle soluble in alcohol, termed "cetrarine;" and common starch; traces of gallic acid and bitartrate of potash are also present. Iodine. Symbol, I. Atomic weight, 126. Iodine is chiefly prepared at Glasgow, from _kelp_, which is the fused ash obtained by burning seaweeds. The waters of the ocean contain minute quantities of the iodides of sodium and magnesium, which are separated and stored up by the growing tissues of the marine plant. In the preparation, the mother-liquor of kelp is evaporated to dryness and distilled with sulphuric acid; the hydriodic acid first liberated is decomposed by the high temperature, and fumes of iodine condense in the form of opaque crystals. _Properties._--Iodine has a bluish-black color and metallic lustre; it stains the skin yellow, and has a pungent smell, like diluted chlorine. It is extremely volatile when moist, boils at 350°, and produces dense violet-colored fumes, which condense in brilliant plates. Specific gravity 4·946. Iodine is very sparingly soluble in water, 1 part requiring 7000 parts for perfect solution: even this minute quantity however tinges the liquid of a brown color. Alcohol and ether dissolve it more abundantly, forming dark-brown solutions. Iodine also dissolves freely in solutions of the alkaline iodides, such as the iodide of potassium, of sodium, and of ammonium. _Chemical Properties._--Iodine belongs to the chlorine group of elements, characterized by forming acids with hydrogen, and combining extensively with the metals (see chlorine). They are however comparatively indifferent to oxygen, and also to each other. The iodides of the alkalies and alkaline earths are soluble in water; also those of iron, zinc, cadmium, etc. The iodides of lead, silver, and mercury are nearly or quite insoluble. Iodine possesses the property of forming a compound of a deep blue color with starch. In using this as a test, it is necessary first to liberate the iodine (if in combination), by means of chlorine, or nitric acid saturated with peroxide of nitrogen. The presence of alcohol or ether interferes to a certain extent with the result.