The simplest way to investigate the study of the gas interchange that takes place in the lungs, between the air and the blood, is to compare the composition of the expired air with that of the atmosphere, and from the alteration found to have taken place during the tidal current we arrive at the changes which the air undergoes during its journey in and out of the air passages, and we then examine the venous and arterial blood in order to ascertain the changes the blood undergoes in becoming arterial.

The atmosphere is made up of a mixture of nitrogen and oxygen with a variable amount of moisture and a minute proportion of carbonic acid.

The following table gives the volume* of the gases in dried air: -

Oxygen

20.96 per cent

, or about 21 per cent.

Nitrogen, . . .

.... 79.02 "

" 79 "

Carbonic dioxide, . .

. . 0.02 - 0.06 "

" 4 parts in 10,000.

The amount of moisture contained in the air is very variable, and depends in a great measure upon the temperature and the direction of the wind. The dampness of the air depends upon the temperature, so that air containing the same absolute amount of moisture may be relatively dry or damp, according as the temperature rises or falls. As a general rule the air is relatively dry, that is to say, it does not contain so much moisture as it is capable of taking up in the form of aqueous vapor at its ordinary temperature. At certain times of the day the air may be saturated, owing to a sudden fall of temperature.

The temperature of the air which we breathe varies considerably, according to the season of the year, etc., but almost always in this country it is lower than that of our bodies.

* On account of the difference in the atomic weights, the atmosphere being only a mechanical mixture of the gases, the proportion by weight is slightly different, being about - Oxygen 23 per cent., Nitrogen 77 per cent.