The power of any given substance to transmit heat varies with the nature of the heat or of its source. For example, glass, water or alum allows the sun's luminous heat rays to pass, while absorbing nearly all of the heat rays from a vessel filled with boiling water. In other words, these substances are diathermanous for luminous rays, but athermanous for obscure rays. (a.) A solution of iodine in carbon di-sulphide transmits obscure rays but absorbs luminous rays. By means of these substances, luminous and obscure rays may be sifted or separated from each other. Dry air is highly diathermanous; watery vapor is highly athermanous for obscure rays.