382 ANIMAL PHYSIOLOGY. 
1. Whatever the condition of the inspired air, that expired 
is about saturated with aqueous vapor—i. e., it contains all that 
it is capable of holding at the existing temperature, 
2. The temperature of the expired air is about that of the 
blood itself, so that if the air is very cold when breathed, the 
body loses a great deal of its heat in warming it. The expired. 
air of the nasal passages is slightly warmer than that of the 
mouth. 
3. Experiment shows that the expired air is really dimin- 
ished in volume to the extent of from one fortieth to one fiftieth 
of the whole. Since two volumes of carbonic anhydride require 
for their composition two volumes of oxygen, if the amount of 
the former gas expired be not equal to the amount of oxygen 
inspired, some - a latter must have been used to form other 
combinations. *, amounting to rather less than 1, is called 
oe 3 
the respiratory coefficient. 
4, The difference between inspired and expired air may be 
gathered from the following: 
Oxygen. Nitrogen. Carbonic dioxide. 
Inspired air........... me sir 20°810° 79°150 0-040 
Expired air................ 16-083 79°587 4:380 
From which the most important conclusions to be drawn 
are, that the expired air is poorer in oxygen to the extent of 
4 to 5 per cent, and richer in carbonic anhydride to somewhat 
less than this amount. 
From experiment it has been ascertained that the amount 
of carbonic dioxide is for the average man 800 grammes (406 
litres, equivalent to 2181 grammes carbon) daily, the oxygen 
actually used for the same period being 700 grammes. But 
the variations in such cases are very great, so that these num- 
bers must not be interpreted too rigidly. Experience proves 
that, while chemists often work in laboratories in which the 
percentage of carbonic anhydride (from chemical decomposi- 
tions) reaches 5 per cent, an ordinary room in which the amount 
of this gas reaches 1 per cent is entirely unfit for occupation. 
This is not because of the amount of the carbon dioxide pres- 
ent, but of other impurities which seem to be excreted in pro- 
portion to the amount of this gas, so that the latter may be 
taken as a measure of these poisons. 
What these are is as yet almost entirely unknown, but that- 
they are poisons is beyond doubt. Small effete particles of 
