tion of balloons, but usually the much cheaper coal gas is
substituted for it. Even hot air is often used when the duration of
ascension is very short. It has been used also as a source of heat and
light in the oxyhydrogen blowpipe. Where the electric current is
available, however, this form of apparatus has been displaced almost
entirely by the electric light and electric furnace, which are much more
economical and more powerful sources of light and heat.

EXERCISES

1. Will a definite weight of iron decompose an unlimited weight of
steam?

2. Why is oxygen passed through the inner tube of the oxyhydrogen
blowpipe rather than the outer?

3. In Fig. 14, will the flame remain at the mouth of the tube?

4. From Fig. 15, suggest a way for determining experimentally the
quantity of water formed in the reaction.

5. Distinguish clearly between the following terms: oxidation,
reduction, combustion, and kindling temperature.

6. Is oxidation always accompanied by reduction?

7. What is the source of heat in the lime light? What is the exact use
of lime in this instrument?

8. In Fig. 12, why is it necessary to dry the hydrogen by means of the
calcium chloride in the tube X?

9. At what pressure would the weight of 1 l. of hydrogen be equal to
that of oxygen under standard conditions?

10. (a) What weight of hydrogen can be obtained from 150 g. of
sulphuric acid? (b) What volume would this occupy under standard
conditions? (c) The density of sulphuric acid is 1.84. What volume
would the 150 g. of the acid occupy?

11. How many liters of hydrogen can be obtained from 50 cc. of sulphuric
acid having a density of 1.84?

12. Suppose you wish to fill five liter bottles with hydrogen, the gas
to be collected over water in your laboratory, how many cubic
centimeters of sulphuric acid would be required?

CHAPTER IV

COMPOUNDS OF HYDROGEN AND OXYGEN; WATER AND HYDROGEN DIOXIDE

WATER

~Historical.~ Water was long regarded as an element. In 1781 Cavendish
showed that it is formed by the union of hydrogen and oxygen. Being a
believer in the phlogiston theory, however, he failed to interpret his
results correctly. A few years later Lavoisier repeated Cavendish's
experiments and showed that water must be regarded as a compound of
hydrogen and oxygen.

~General methods employed for the determination of the composition of a
compound.~ The composition of a compound may be determined by either of
two general p