Potassium iodide. Light micrograph of crystals of potassium iodide which is used in photography and medicine. This chemical is a white crystalline compound which is very soluble in water. It does not occur naturally but can be manufactured in a reaction between iodine and a hot solution of caustic soda. It is used in photography for preparing gelatin emulsions and is used in medicine to treat rheumatism and over-active thyroid glands. It is also used to dissolve iodine to aqueous solutions. Magnification: x8 at 35mm size.