3 12 
Journal of Agricultural Research 
Vol. V, No. 8 
volume of total air present the sum of the volumes of carbon dioxid and 
water vapor present, as shown by analyses of the residual air, and the 
volume of nitrogen, including that present at the beginning of the period 
and that added with the oxygen admitted during the period, due allow¬ 
ance being made in the latter for any impurity. 
To compute the total quantities of carbon dioxid and water vapor in 
the air of the chamber, the volumes corresponding to the weights of the 
gases removed by the small absorber system from the air sample meas¬ 
ured by the meter are multiplied by a factor representing the ratio 
between the volume of the sample and the total volume of air in the 
chamber when both are reduced to standard conditions of temperature 
(o° C.) and of pressure (760 mm. of mercury). The necessity for accuracy 
in the analysis of the sample is shown by the fact that under usual experi¬ 
mental conditions there are more than 4,000 liters of air in the chamber; 
hence, any error in the determination of the quantities of water vapor 
and carbon dioxid in a 10-liter sample is multiplied over 400 times. 
The actual volume of air in the chamber under standard conditions 
depends upon the capacity of the chamber and the barometric pressure 
and temperature of the air in it. These factors must be accurately 
determined, since a difference of 1 mm. in the pressure means a differ¬ 
ence of over 5 liters in the computation of the actual volume of gas, 
while a difference of 1 degree in the temperature means a difference of 
about 15 liters in the total volume. An error in these determinations 
has some effect upon the computation of the quantities of residual gases, 
though the effect of any error likely to occur upon the quantity of water 
vapor would be quite insignificant, as there are seldom more than 90 
liters present, and commonly less. The effect on the computation of 
carbon dioxid would be somewhat larger, as there might be in some cir¬ 
cumstances 100 liters or more in the air; but under ordinary conditions 
the quantity is decidedly less, and the error would be relatively unim¬ 
portant. The effect would be greatest upon the computation of the 
quantity of oxygen, as under normal conditions there could be as much 
as 850 liters present. 
MEASUREMENT OF CAPACITY OF THE CHAMBER 
The capacity of the chamber is known very accurately. It may be 
computed from the dimensions of the chamber, and it may be directly 
ascertained by determining the proportion of oxygen in the well-stirred 
air of the sealed chamber before and after the admission of a known 
volume of the gas. 
measurement of barometric pressure of the air 
The barometric pressure of the air of the chamber, which, because of 
the air-tension equalizer mentioned on page 304, fluctuates the same as 
