Optic projection : principles, installation and use of the magic lantern, projection microscope, reflecting lantern, moving picture machine (1914)

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CH. XU DANGER OF FIRE 431 DANGER OF FIRE ยง 596. Before the introduction of non-inflammable films, all films were made by coating the emulsion upon celluloid. This is a nitrate (the trinitrate) of cellulose to which is added a certain amount of camphor. A more highly nitrated cellulose is called gun cotton. There is sufficient oxygen in the nitrated cellulose to partially support combustion and it is the cause of the highly inflammable nature of celluloid. This was strikingly shown in some experiments made to ascertain the possible danger from an ignited film. A small reel of film was lighted and put under a tin box so that no air could get at it. A fire in ordinary combustibles, such as paper or wood, would soon be smothered, but the roll of film continued to decompose in the closed box. This shows that if a roll of film, even in a closed fire proof magazine, once catches fire it will continue to burn as long as there is anything left of it. The gases given off from the film decomposing in a closed box are very disagreeable and will burn in contact with air if they are once lighted. If celluloid will burn so vigorously in a closed box, what would be the effect of a large reel of film lying uncoiled in a waste basket or on the floor should it once catch fire ? This was the practise in the early days of the art of projecting moving pictures. Seven to ten meters (twenty or thirty feet) of film piled loosely, will be completely consumed in a few seconds, burning with a fierce flair, e while it lasts. In view of this very evident danger, modern apparatus is designed to make it as safe as possible. To the good design of the machine must be added the cooperation of the operator to prevent a fire. The fire shutter (fig. 228), automatically closes whenever the machine is not running. This shutter is placed in front of the film and prevents the light of the arc from striking it except when it is in motion. If the film should break, however, a tag end might remain in the aperture plate and be ignited, the fire shutter remaining open while the crank was being turned. To prevent this trouble the light should be instantly shut off whenever a film breaks.