Envelope X-RAY Spray turns opaque paper temporarily translucent, allowing the user to view the contents of an envelope without ever opening it. 30 seconds after the application, the envelope will return to its original state, leaving absolutely no markings, discoloration, or other indications of use. Each can treats several hundred square inches. Non-conductive and non-photochemically reactive. Environmentally-friendly (contains no Freon). Size 6.25" x 2". Net weight: 5 oz. WARNING: Not to be used on U.S. Mail, except by, or with the express permission of, the addressee. Not shippable by Air.