he end of this mark a hot poker. When the glass cracks, lead the
crack around the bottle by heating about one half inch in advance of it.)
Place the bottle in a large glass jar filled two thirds full of water
(Fig. 52). Let the space above the water represent the chest cavity and
the water surface represent the diaphragm. Raise the bottle, noting that
the water falls, thereby increasing the space and causing air to enter.
Then lower the bottle, noting the opposite effect. To show the movement of
the air in and out of the bottle, hold with the hand (or arrange a support
for) a burning splinter over the mouth of the bottle.

*To estimate the Capacity of the Lungs.*--Breathing as naturally as
possible, expel the air into a spirometer (lung tester) during a period,
say of ten respirations (Fig. 53). Note the total amount of air exhaled
and the number of "breaths" and calculate the amount of air exhaled at
each breath. This is called the _tidal_ air.

[Fig. 53]

Fig. 53--*Apparatus* (spirometer) for measuring the capacity of the lungs.

2. After an ordinary inspiration empty the lungs as completely as possible
into the spirometer, noting the quantity exhaled. This amount, less the
tidal air, is known as the _reserve_ air. The air which is now left in the
lungs is called the _residual_ air. On the theory that this is equal in
amount to the reserve air, calculate the capacity of the lungs in an
ordinary inspiration.

3. Now fill the lungs to the full expansion of the chest and empty them as
completely as possible into the spirometer, noting the amount expelled.
This, less the tidal air and the reserve air, is called the _complemental_
air. Now calculate the total capacity of the lungs.

CHAPTER VIII - PASSAGE OF OXYGEN THROUGH THE BODY

What is the nature of oxygen? What is its purpose in the body and how does
it serve this purpose? How is the blood able to take it up at the lungs
and give it off at the cells? What becomes of it after being used? These
are questions touching the maintenance of life and they deserve careful
consideration.

*Nature of Oxygen.*--To understand the relation which oxygen sustains to
the body we must acquaint ourselves with certain of its chemical
properties. It is an element(44) of intense affinity, or combining power,
and is one of the most active of all chemical agents. It is able to
combine with most of the other elements to form chemical compoun