le as with oxygen, for in the 1,300
liters of air in the chamber there are some 250 liters of oxygen, and
slight changes in the composition of this air indicate considerable
changes in the amount of oxygen. Great changes may also take place in
the amounts of carbon dioxide and water-vapor under certain conditions.
In some experiments, particularly where there are variations in muscular
activity from period to period, there may be a considerable amount of
carbon dioxide in the residual air and during the next period, when the
muscular activity is decreased, for example, the percentage composition
of the air may vary so much as to indicate a distinct fall in the amount
of carbon dioxide present. Under ordinary conditions of ventilation
during rest experiments the quantity of carbon dioxide present in the
residual air is not far from 8 to 10 grams. There are usually present in
the air not far from 6 to 9 grams of water-vapor, and hence this
residual amount can undergo considerable fluctuations. When it is
considered that an attempt is made to measure the total amount of carbon
dioxide expired in one hour to the fraction of a gram, it is obvious
that fluctuations in the composition of residual air must be taken into
consideration.

It is extremely difficult to get a fair sample of air from the chamber.
The air entering the chamber is free from water-vapor and carbon
dioxide. In the immediate vicinity of the entering air-tube there is air
which has a much lower percentage of carbon dioxide and water-vapor than
the average, and on the other hand close to the nose and mouth of the
subject there is air of a much higher percentage of carbon dioxide and
water-vapor than the average. It has been assumed that the composition
of the air leaving the chamber represents the average composition of the
air in the chamber. This assumption is only in part true, but in rest
experiments (and by far the largest number of experiments are rest
experiments) the changes in the composition of the residual air are so
slow and so small that this assumption is safe for all practical
purposes.

Another difficulty presents itself in the matter of determining the
amount of carbon dioxide and water-vapor; that is, to make a
satisfactory analysis of air without withdrawing too great a volume from
the chamber. The difficulty in analysis is almost wholly confined to the
determination of water-vapor, for while there are a large number of
methods for determ