r contains carbon
dioxid_. Put a glass tube into a bottle of lime water and breathe
through the tube. The A liquid will soon become cloudy, because the
carbon dioxid of the expired air throws down the lime held in solution.

Experiment 121. "A substitute for a clinical thermometer may be readily
contrived by taking an ordinary house thermometer from its tin case, and
cutting off the lower part of the scale so that the bulb may project
freely. With this instrument the pupils may take their own and each
other's temperatures, and it will be found that whatever the season of
the year or the temperature of the room, the thermometer in the mouth
will record about 99 degrees F. Care must, of course, be taken to keep
the thermometer in the mouth till it ceases to rise, and to read while
it is still in position."--Professor H. P. Bowditch.

Experiment 122. _To illustrate the manner in which the movements of
inspiration cause the air to enter the lungs._ Fit up an apparatus, as
represented in Fig. 95, in which a stout glass tube is provided with a
sound cork, B, and also an air-tight piston, D, resembling that of an
ordinary syringe. A short tube, A, passing through the cork, has a small
India-rubber bag, C, tied to it. Fit the cork in the tube while the
piston is near the top. Now, by lowering the piston we increase the
capacity of the cavity containing the bag. The pressure outside the bag
is thus lowered, and air rushes into it through the tube, A, till a
balance is restored. The bag is thus stretched. As soon as we let go the
piston, the elasticity of the bag, being free to act, Movements of
drives out the air just taken in, and the piston returns to its former
place.

[Illustration: Fig. 95. Apparatus for Illustrating the Movements of
Respiration.]

It will be noticed that in this experiment the elastic bag and its tube
represent the lungs and trachea; and the glass vessel enclosing it, the
thorax.

For additional experiments on the mechanics of respiration, see Chapter
XV.

Chapter IX.

The Skin and the Kidneys.

232. The Elimination of Waste Products. We have traced the food from
the alimentary canal into the blood. We have learned that various food
materials, prepared by the digestive processes, are taken up by the
branches of the portal vein, or by the lymphatics, and carried into the
blood current. The nutritive material thus absorbed is con