Man's physical universe

xanabras

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?

More magazines by this user
Similar magazines