312 Journal of Agricultural Research voi. v. No. s 



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 lo-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 i 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 i 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 OP 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 



