5S4 MAN IS MASTERING HIS MATERIAL WORLD silver chloride. A mixture of the bromide and iodide (within certain limits) is more sensitive than either the pure bromide or iodide. Photographic films which are very sensitive to light are called fast films because they can be used with a fast shutter speed (short exposure). A slow film, on the other hand, requires long exposures. Suspensions of these salts in specially prepared gelatine may be made extremely sensitive to infrared, ultraviolet, and other portions of the electromagnetic spectrum by the addition of special sensitizing agents and by heat treatment of the emulsion. One of the first observations which led to the modern sensitized films was based on the fact that films whose emulsion contained gelatine made from cows which had been eating plants containing mustard oil were unusually sensitive. This observation led to the use of mustard oil (allyl isothiocyanate) to sensitize emulsions. The emulsion may be coated on celluloid or cellulose nitrate or acetate film bases, glass plates, or paper. After exposure to light, the film or plate is placed in the developing bath, which contains one or more of a number of substances which will reduce the exposed areas, liberating free silver from its salts. The size of the silver particles and other factors determine the grain, which is very important in enlarging. Developers which produce very small free silver particles are called fine-grain developers. The speed and contrast in developing are controlled by the use of substances such as sodium carbonate (the accelerator), to provide an alkaline solution, and potassium bromide (the restrainer), which helps to prevent reduction of the unactivated silver salts. Sodium sulfite (the preservative) is also added to the developing solution to prevent deterioration due to the action of the reducing agent with the oxygen of the air. The unexposed compounds remain unchanged after the reduction of the exposed portion. These unchanged compounds would also be reduced if left exposed to the light, so they are treated with "hypo" (sodium thiosulfate) in the fixing bath, which dissolves these unreduced silver salts. An acid is generally added to the fixing bath to stop the action of the developer because it neutralizes the alkali which is essential to the action of the developer. Potassium or chrome alum is added to harden the gelatine of the emulsion, which is softened and swollen by the sodium carbonate or other alkaline substance in the developing solution. Sodium sulfite is also added to oppose the decomposition of the "hypo." Ordinary films and plates when developed are called negatives. Negatives represent the reverse of the light values of the original subject. By passing light through a negative held in contact with a paper, film, or plate coated with photographic emulsion, the tone
THE NATURE OE MOLECULES 585 values are again reversed. This operation is called contact printing. If a lens is placed between the negative and the photographic printing paper, an enlarged or reduced image is obtained. A device which makes it possible to vary the size of the image produced by changing the relative distance between the lens, the negative, and the unexposed emulsion surface is called an enlarger. The process of developing and fixing the paper, film, or plate which has been exposed by contact or in an enlarger will produce a faithful reproduction of the original subject. Amateur motion-picture films do not need to be printed, the original film being treated by a reversal process of developing which produces a positive rather than a negative. In addition to processes of developing and fixing, there are the processes of toning, in which the black silver is changed to a salt of another color, and the processes of reduction and intensification, which are used to correct under- and overexposed negatives. STUDY QUESTIONS 1. What is meant by valence? What rule of valence must be observed in writing formulas of compounds? 2. How do you account for positive and negative valence? 3. Define metals and nonmetals in terms of their tendency to gain or lose electrons. 4. Compare electrovalent reactions with covalent reactions. 5. What are ions, and how are they formed? 6. Why are free ions not found when electrovalent compounds are dissolved in some solvents, while they are found in solution in certain other solvents, such as water? 7. Outline the main principles of photography. 8. Why is activation important for covalent compounds? 9. What type of compound is largely covalent in nature? 10. How may molecules be activated? n. Explain the difference between exothermic and endothermic compounds. 12. Why do endothermic reactions require a continual supply of heat to keep them acting? 13. What is meant by developing a photographic film? 14. What is the purpose of fixing a photographic film? 15. What is the basic principle of photography?