THE RESPIRATORY PROCESS 
123 
G. THE DEMONSTRATION OF THE NATURE OF EX¬ 
PIRED AIR. 
1. The Excess of Carbon Dioxide. —This may be demon¬ 
strated by two bottles connected with a mouthpiece which is so 
arranged that the inhaled air passes through lime water in one 
bottle and the exhaled air through lime water in the other. This 
arrangement is made by using as a mouthpiece a Y-tube, one 
arm of which is connected by means of rubber tubing with a glass 
tube which passes through the tightly fitting cork of one bottle, 
reaching nearly to the bottom of the bottle, while the other arm 
of the tube is connected with a glass tube which leads only a short 
distance below the cork fitted tightly into the other bottle. The 
first bottle has an open glass tube leading through its cork from 
the upper part of the bottle, while the second bottle has another 
glass tube leading from near the bottom of the bottle. Each 
bottle is filled half full with filtered lime water at the beginning 
of the experiment. A rubber tube attached to the stem of the Y- 
tube serves as a mouthpiece, and can be washed or renewed as 
needed. Breathe in and out through this apparatus for a short 
time and note the result. Milkin ;ss induced in lime water by a 
stream of gas passing through it is a test for carbon dioxide in the 
gas. Record the result , accompanying it by a diagram of the appara¬ 
tus. Finally empty the lime water out and thoroughly rinse each 
bottle, using a weak solution of hydrochloric acid if necessary. 
2. The Deficiency of Oxygen. —Invert over water, with its 
mouth submerged, a wide-mouthed bottle filled with air. By 
means of a bent tube passed under water and into the bottle 
breathe the contained air in and then out two or three times. Lift 
the bottle and, keeping its mouth down, quickly put a lighted taper 
into it. As a control burn a taper in a similar inverted bottle filled 
with air. A flame will not burn in an atmosphere of less than 17% 
of oxygen, i.e. } a tension equal to 129 mm. of mercury. Record 
your conclusions. 
