18 ORGANISM AND ENVIRONMENT 
Some of the immediate practical applications of the 
new knowledge with regard to the regulation of 
breathing are perhaps of sufficient interest to be men- 
tioned shortly. The air of all sorts of confined spaces 
is apt to be vitiated by the presence of CO,; and along 
with the excess of CO, there is usually a deficiency of 
oxygen, since the vitiation is due to processes of oxi- 
dation; in which oxygen is used up in proportion as 
CO, is formed. In the air of ordinary rooms CO, 
is formed and oxygen used up by respiration and by 
the burning of illuminants. The natural ventilation 
of an ordinary room is, however, so considerable that 
it is very seldom that the percentage of CO, in the 
air exceeds 0.5 per cent. What effects will the gaseous 
impurity in such air have? Clearly none that are 
appreciable. The breathing will be very slightly 
deeper, so as to keep the alveolar CO, percentage con- 
stant ; but the increase in breathing will be less than a 
tenth, and such an increase is totally unappreciable 
subjectively. The slightly increased breathing will 
also keep the oxygen percentage in the alveolar air 
from falling, so that the diminished oxygen percent- 
age in the air will be of no account. We must thus 
seek elsewhere than in the gaseous impurities of the 
air of rooms for the causes of the discomfort felt in 
crowded rooms. 
In mines and other underground spaces the propor- 
tion of CO, often goes much higher, and may reach 
about 3 per cent in places where a light will still burn. 
With 3 per cent of CO, in the air the breathing is 
doubled. This effect becomes just noticeable during 
