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286 Transactions of S.M.P.E., August 1927
toxicity of impure samples of tetrachloride, a 50% mixture by volume with gasoUne reducing this considerably. This mixture burns with great difficulty and is satisfactory from a fire hazard standpoint although the proportion of the two liquids necessary to give a noninflammable mixture depends on the nature of the gasoHne. It is considered that pure carbon tetrachloride is to be preferred to such a mixture for general purposes.
Film Moistening Liquids.
In addition to accumulating oil during projection, both the film base and gelatin coating lose moisture and tend to become brittle owing to the excessive heat to which the film is subjected. If the film were allowed to cool to room temperature between successive projections, little trouble would be encountered, but in practice the film does not cool off sufficiently between successive projections and the resulting baking process drives out the moisture, which results in brittleness.
If film which has been rendered brittle in this manner is exposed to a moist atmosphere even for only a relatively short time it tends to regain its flexibility. It is not possible to do this by placing the tightly wound reels of film in a humidor or a vessel containing water because the moisture penetrates the convolutions of film very slowly. It would be possible to humidify the film satisfactorily by passing it continuously through a humid chamber or by winding the film in contact with a damp strip of paper or other absorbent ribbon. Such a system however, is inconvenient in the theatre or exchange.
A satisfactory method of moistening film is to immerse it in a mixture of water and a water-miscible volatile liquid such as grain alcohol. The percentage of water to be used in the mixture depends on the degree of brittleness of the film and the time which elapses between application and evaporation of the liquid. If an application machine of the Dworsky^ type is used, this depends on the rate of passage of the film through the machine. During this short period Uttle or no swelling of the gelatin coating occurs, but sufficient moisture is absorbed to restore the flexibility of the dried out gelatin coating. Moreover, when the film is wound up in a roll, the dried out film base can also absorb moisture by virtue of being in contact with the moistened emulsion. Fihn base absorbs moisture relatively slowly so that little or none is absorbed by it in the period of apphcation of the moistening liquid.