nd in its way. Among these agencies are heat, light, and
electricity. As a rule, solution also promotes action between two
substances. Sometimes these agencies may overcome chemical attraction
and so occasion the decomposition of a compound.

EXERCISES

1. To what class of changes do the following belong? (a) The melting
of ice; (b) the souring of milk; (c) the burning of a candle; (d)
the explosion of gunpowder; (e) the corrosion of metals. What test
question must be applied in each of the above cases?

2. Give two additional examples (a) of chemical changes; (b) of
physical changes.

3. Is a chemical change always accompanied by a physical change? Is a
physical change always accompanied by a chemical change?

4. Give two or more characteristics of a chemical change.

5. (a) When a given weight of water freezes, does it absorb or evolve
heat? (b) When the resulting ice melts, is the total heat change the
same or different from that of freezing?

6. Give three examples of each of the following: (a) mechanical
mixtures; (b) chemical compounds; (c) elements.

7. Give the derivation of the names of the following elements: thorium,
gallium, selenium, uranium. (Consult dictionary.)

8. Give examples of chemical changes which are produced through the
agency of heat; of light; of electricity.

CHAPTER II

OXYGEN

~History.~ The discovery of oxygen is generally attributed to the English
chemist Priestley, who in 1774 obtained the element by heating a
compound of mercury and oxygen, known as red oxide of mercury. It is
probable, however, that the Swedish chemist Scheele had previously
obtained it, although an account of his experiments was not published
until 1777. The name oxygen signifies acid former. It was given to the
element by the French chemist Lavoisier, since he believed that all
acids owe their characteristic properties to the presence of oxygen.
This view we now know to be incorrect.

~Occurrence.~ Oxygen is by far the most abundant of all the elements. It
occurs both in the free and in the combined state. In the free state it
occurs in the air, 100 volumes of dry air containing about 21 volumes of
oxygen. In the combined state it forms eight ninths of water and nearly
one half of the rocks composing the earth's crust. It is also an
important constituent of the compounds which compose plant and animal
tissues; for example, about 66% by weight of the human body is oxygen.

~Preparation.~ Al