L. H. GRAY 



flow is 130 ml./min. The zero is constant over short periods of time to about 

 + 0-01 [J.A. This apparatus is adapted to the measurement of quantities of 

 oxygen in the manner previously outlined'* by the inclusion of the fluxmeter, 

 F, in series with the galvanometer in the balancing arm of the potentio- 

 meter [Figure 2). The oxygen content of the sample introduced into the 

 analysing compartment is collected during a period of 2 min and the amount, 

 if small, is proportional to the fluxmeter reading. The absolute amount of 

 oxygen in the sample is obtained by reference to the fluxmeter reading which 

 results from the introduction by electrolysis of a known amount of oxygen 

 into the system. Replicate observations showed that the oxygen content of 

 a 5 [j.1. bubble at a partial pressure of up to 10 mm mercury could be mea- 

 sured in this way to + 10"^^ g corresponding to +0-1 mm mercury in 

 partial pressure. The overall errors with which differences in partial pressure 

 between gas and fluid phase could be measured by the formation and 



2H2+O2 

 Catalyst 



^ 



H. 



\^ 



Flowmeter 



Electrolysis 

 cell (calibration) 



Hersch cell 



A - Microammeter 

 F- Fluxmeter 

 G Galvanometer 



Sample 



Potentiometer 



Figure 2. Apparatus for the micro-assay of oxygen 



retraction of a bubble under fluid which contained tumour cells in the 

 manner described above, were about twice those stated above. Check 

 measurements with tumour-bearing fluid at difTerent temperatures and 

 diflferent cell concentrations showed approximately: (a) zero diflferences in 

 partial pressure between gas and fluid when cellular respiration was poisoned 

 by calcium cyanide, (b) proportionality between the pressure differential 

 and the concentration of tumour cells, and (c) that the pressure differential 

 at different temperatures was roughly in proportion to the measured Qq^ 

 of the cells at these temperatures. On the basis of all these measurements, it 

 was possible to conclude that when 4 ml. of tumour-bearing fluid was 

 equilibrated with gas in the apparatus shown in Figure 1, and in the manner 

 described, the pressure differential at 18°C was 0-4 mm mercury at a tumour 

 cell concentration of 4 x 10'' cells/ml. It was, therefore, estimated that in the 

 irradiation experiments the concentration of dissolved oxygen in the fluid 

 phase was • 7 [j.M/1. less than that which would correspond with true 

 equilibrium between gas and fluid phases in the absence of cellular respir- 

 ation. This small diflference was allowed for. 



79 



