402 ENERGY MAY BE PROPAGATED BY VIBRATIONS Special incandescent lamps, made of glass that permits the passage of ultraviolet rays but absorbs the visible light, are also available. The life of these lamps is short because of the high temperatures required for ultraviolet production with filament sources. More intense sources of ultraviolet rays use either an electric arc or a mercury-vapor lamp with a suitable filter. Modern photofiood lights produce radiations so rich in ultraviolet rays that several of these lights, placed in a box with a window made of a kind of glass that filters out all but the ultraviolet radiations, provide an inexpensive source of ultraviolet rays. The Wave Lengths of Ultraviolet Light Sources Vary. The wave lengths of ultraviolet radiations may range from 136 angstroms to 4000 angstroms. The wave lengths of ultraviolet rays may be determined by chemical reactions which are influenced by i certain range of wave lengths only. Fig. 175. Germicidal lamps used to sterilize glasses in a cafeteria. (Courtesy of the Westinghouse Electric and Manufacturing Company.) Thus the range 2200-3200 A. liberates iodine from potassium iodide, potassium nitrate is reduced to potassium nitrite by a range of 2200- 4000 A., and methylene blue is decolorized by light of 2500-4000 A. The wave length most effective against bacteria is about 2600 A. Radiations between 3300 and 4000 A. do not affect the body and do not have germicidal action, while the shorter wave lengths produce "sunburns" and conjunctivitis of the eyeball. The wave lengths emitted by different ultraviolet-ray sources depend upon the temperature of the radiating sources, the nature of the glass in the lamp, the nature of the filter, and other factors.
LIGHT MAY BE PRODUCED BY LUMINESCENCE 403 Germ-killing Lamps Are Ultraviolet Sources. Germ-killing lamps, such as the "Sterilamp," "Sterilaire," and General Electric "Germicidal" lamps, use a very thin layer of glass of a type that transmits a high percentage of ultraviolet light. Most lamps of this type operate on low current, about as much as a Christ-' mas-tree light, and develop very little heat. The wave lengths of the ultraviolet radiations produced by such lamps are deadly to bacteria; such radiations cause temporary, but very painful conjunctivitis of the eyeball. Goggles may be worn to protect the eyes against these radiations. Germ-killing sources have been used in refrigerators and display cases to keep foods such as meat fresh much longer, and they have improved the health of cows in dairies and lowered the death rate of chicks in brooders by reducing airborne contamination. They sterilize the air in operating rooms, thus decreasing the possibilities of infections. Wounds may be kept sterile until they heal, and skin infections may be killed. Such sources may be used in restaurants to sanitize drinking glasses and other utensils. In general, this source of mold-, fungi-, and bacteria-killing ultraviolet light is in many fields the most important prophylactic germicidal agent now available. r ' *