mixed in the proportion in which they
exist in the atmosphere, heat is neither evolved nor absorbed by the
process. What important point does this suggest?

2. What essential constituent of the air is found in larger amount in
manufacturing districts than in the open country?

3. Can you suggest any reason why the growth of clover in a field
improves the soil?

4. Why are the inner walls of a Dewar bulb sometimes coated with a film
of silver?

5. To what is the blue color of liquid air due? Does this color increase
in intensity on standing?

6. When ice is placed in a vessel containing liquid air, the latter
boils violently. Explain.

7. Taking the volumes of the oxygen and nitrogen in 100 volumes of air
as 21 and 78 respectively, calculate the percentages of these elements
present by weight.

8. Would combustion be more intense in liquid air than in the gaseous
substance?

9. A tube containing calcium chloride was found to weigh 30.1293 g. A
volume of air which weighed 15.2134 g. was passed through, after which
the weight of the tube was found to be 30.3405 g. What was the
percentage amount of moisture present in the air?

10. 10 l. of air measured at 20 deg. and 740 mm. passed through lime water
caused the precipitation of 0.0102 g. of CaCO_{3}. Find the number of
volumes of carbon dioxide in 10,000 volumes of the air.

CHAPTER IX

SOLUTIONS

~Definitions.~ When a substance disappears in a liquid in such a way as to
thoroughly mix with it and to be lost to sight as an individual body,
the resulting liquid is called a _solution_. The liquid in which the
substance dissolves is called the _solvent_, while the dissolved
substance is called the _solute_.

~Classes of solutions.~ Matter in any one of its physical states may
dissolve in a liquid, so that we may have solutions of gases, of
liquids, and of solids. Solutions of liquids in liquids are not often
mentioned in the following pages, but the other two classes will become
very familiar in the course of our study, and deserve special attention.

SOLUTION OF GASES IN LIQUIDS

[Illustration: Fig. 30]

It has already been stated that oxygen, hydrogen, and nitrogen are
slightly soluble in water. Accurate study has led to the conclusion that
all gases are soluble to some extent not only in water but in many other
liquids. The amount of a gas which will dissolve in a liquid depends
upon a number of conditions, and these can best be understood b