708 Journal of Agricultural Research vol. vi.No. is 



SUPPLYING OXYGEN TO THE AIR 



The oxygen supplied to the chamber to replace that absorbed by the 

 active material is obtained from a cylinder which contains the gas under 

 pressure. It is admitted at such a rate that the apparent volume of gas 

 in the chamber as indicated by the tension equalizer is relatively constant. 

 The regulation may be by hand ; or the tension equalizer may be arranged 

 to cause a valve in the oxygen feed line to be opened or closed as the 

 volume of gas in the system diminishes or increases. The small cylinder 

 with the gas-pressure-reducing valve attached weighs less than 10 kgm., 

 and changes in the weight of it may be ascertained to an accuracy of 0.05 

 gin., which means that the actual volume of the gas admitted is known 

 to within 50 c. c. With regularity in the rate of admission of oxygen, 

 other methods of determining the quantity, as by means of an accurate 

 meter carefully calibrated, or by the filling and emptying of a calibrated 

 spirometer, are suitable. In the latter case a sensitive spirometer could 

 serve also as a tension equalizer. 



CHANGES IN THE COMPOSITION OF THE RESIDUAL AIR 



At the beginning and the end of each experimental period a portion of 

 the air leaving the rotary pump is shunted through a train of small 

 absorbing devices * and then through an accurate meter, which stands on 

 the top shelf of the absorber table. The air leaving the meter is restored 

 to that in the main line returning to the chamber. The weight of each 

 small absorber, which is less than 100 gm., is ascertained to an accuracy 

 of 0.1 mgm. The quantities of water vapor and carbon dioxid in the 

 measured sample of air, as shown by the increase in the weights of the 

 small absorbers, represent very accurately those of the atmosphere of the 

 chamber. With such material as ripening fruit in the chamber, any 

 change in the composition of the atmosphere occurs so slowly that it has 

 no appreciable effect on the air of the chamber during the period of 

 taking the sample. A fan to stir the air is unnecessary, the total volume 

 being small when the quantity of active material used for experimental 

 purposes is inclosed in the chamber. The air is withdrawn from the cham- 

 ber through a pipe terminating at the floor in a cross with both ends open, 

 while air is returned to the chamber through a pipe opening near the 

 ceiling. The circulating air thus traverses the full depth of the chamber. 

 At the usual rate of ventilation the total volume of air in the system com- 

 pletes the circulation several times per hour. 



When a sample of air is desired for the determination of the proportion 

 of oxygen present, it is usually taken from that returning from the 

 absorbers to the chamber, no oxygen being admitted from the cylinder 

 to the system at the time. Ordinarily this determination is not necessary, 

 since by properly accounting for the different products removed from and 

 admitted to the ventilating system, the quantity of oxygen consumed 

 from the atmosphere may be computed. 



1 Langworthy, C. F., and Milner, R. D. Op. cit.. p. 311. 



