tself. The silver bromide is much more sensitive than the
chloride, so that the printing can be done in artificial light.
Since the darkest places on the negative cut off the most
light, it is evident that the lights of the print will be the
reverse of those of the negative, and will therefore correspond
to those of the object photographed. The print is therefore
called the positive.

EXERCISES

1. Account for the fact that copper has been used for so long a time.

2. Write equations for the action of concentrated sulphuric and nitric
acids upon the metals of this family.

3. How would you account for the fact that normal copper sulphate is
slightly acid to litmus?

4. Contrast the action of heat on cupric nitrate and mercuric nitrate.

5. State reasons why mercury is adapted for use in thermometers and
barometers.

6. How could you distinguish between mercurous chloride and mercuric
chloride?

7. Write equations for the preparation of mercuric and mercurous
iodides.

8. How would you account for the fact that solutions of the different
salts of a metal usually have the same color?

9. Crude silver usually contains iron and lead. What would become of
these metals in refining by parting with sulphuric acid?

10. In the amalgamation process for extracting silver, how does ferrous
chloride convert silver chloride into silver? Write equation. Why is the
silver sulphide first changed into silver chloride?

11. What impurities would you expect to find in the copper sulphate
prepared from the refining of silver?

12. How could you prepare pure silver chloride from a silver coin?

13. Mercuric nitrate and silver nitrate are both white solids soluble in
water. How could you distinguish between them?

14. Account for the fact that sulphur waters turn a silver coin black;
also for the fact that a silver spoon is blackened by foods (eggs, for
example) containing sulphur.

15. When a solution of silver nitrate is added to a solution of
potassium chlorate no precipitate forms. How do you account for the fact
that a precipitate of silver chloride is not formed?

CHAPTER XXIX

TIN AND LEAD

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| SYMBOL | ATOMIC | DENSITY | MELTING | COMMON OXIDES
| | WEIGHT | | POINT |
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