The following are the notable characters in the tidal air on its leaving the air passages: i. It is rich in C02, containing in quiet breathing on an average 4.38 per cent, instead of.04 per cent.

2. It is poor in O, containing about 4.5 per cent, less than the atmosphere, i. e., 16.46 per cent.

3. A slight increase in the N has been observed, possibly the outcome of nitrogenous metabolism.

4. The temperature of the air is approximated to that of the body, and it therefore commonly exceeds the temperature of the air inspired. The air on leaving the air passages is about 36.5° C. This is not much influenced by the temperature of the atmosphere, as may be seen from Valentine's Table: -

Temperature of the Atmosphere and of Expired Air.

- 6.3°C. . == +29.8°C.

+ 17.00 C. = + 36.20 c.

+ 44.o°C. = +38-5°C.

It can be seen from the last statement that very hot air (4- 440 C.) if breathed is cooled in its transit through the air passages.

5. In quiet breathing the expired air is saturated with moisture; in rapid breathing this is not the case. It must be remembered that the air when warm is capable of holding a greater quantity of vapor than when it was inspired. The difference can be best appreciated in cold weather, when the vapor of the warm expired air is condensed on meeting the cold atmosphere. Great quantities of water and heat are given off in producing this saturation.

6. If the tidal air be dried and cooled and measured at a certain pressure before and after respiration, it is found that the expired air has lost about 1/50 of its volume. But owing to the expansion from the increased temperature and the presence of the vapor, the volume of air when expired is greater than that inspired.

If the oxygen were all used to make C02, these volumes ought to be the same, for the volume of C02 is equal to that of the O it contains, if set free. The volume C02 given off is, however, only about 4.38 to 4.5 volumes of O taken in, so that part of the O must be used in some other way, probably in forming H20 and urea.

7. The expired air is also said to contain traces of the following impurities: (1) ammonia, (2) hydrogen, (3) carburetted hydrogen (CH1), (4) organic matter. These, and probably other impurities, give the breath its peculiar odor and noxious properties, for an atmosphere rendered "stuffy" by expired air is much more injurious to health than an atmosphere in which a similar deficiency of O or excess of C02 had been artificially produced by chemical means; this fact ought to be remembered when calculating the ventilation required for hygienic purposes. The following table may assist in comparing the atmosphere with the expired air: -

Atmosphere.

Expired Air.

Difference.

CO2................

.04 per cent.

4.38 per cent.

+4.34

0, .......

20.81

16 .03

-4.78

N,.......

79.15 "

79.55 "

+ .40

Temperature, . .

-6° C - + 250 C.

29.8° C.- 38.50 C.

Moisture, ....

about 10 grms. to 1 cubic metre.

about 40 grms. to 1 cubic metre.

Volume ....................

......................

apparently increased, ab-solutely reduced 1/50

Impurities, . . .

....

NH3, H, CH1, and poison-ous organic matter.

About one-seventh of the O which is used does not take part in the production of the C02, but this proportion may vary greatly. Thus, the estimation of the C02 can give no sure guide to the amount of O taken up; and each gas has to be estimated separately if an accurate measurement be required.

The average amount per diem may be said to be : -

Carbon dioxide given off about.......800 grammes.

Oxygen consumed " .......700 "

Water given off " .......500 "

The amounts of O taken up and of C02 given off differ in 30 different individuals and in the same individuals under varying circumstances, among which the following may be enumerated: - 1. Increase in the rapidity or the depth of respiratory movements, accompanied by an increase in the tidal stream, produces an increase of the total amount of C02 given off, while the percentage in the volume of expired air is diminished.

2. It varies with age. The amount increases with age up to 30 years, and then remains constant.

3. Sex; is less in women than in men, but it increases in pregnancy.

4. With muscular activity it is notably increased.

5. Change of temperature of the air has a marked influence on the C02 output of cold-blooded animals, which is increased in direct proportion to the elevation of temperature. The effect on warm-blooded animals is the opposite, so long as they can regulate their temperature. The sustentation of the body temperature in cold weather is accompanied by a distinct increase in the output of carbon dioxide.

6. The time of day: a maximum is arrived at about midday and a minimum about midnight.

7. An increase in the amount of carbon dioxide in the atmosphere diminishes the amount given off from the lungs.