134 MEASUREMENTS OF GAS EXCHANGE 



vessels while they are submerged. The shaking mixes the biological ma- 

 terial thoroughly and assures the even distribution of gases within the 

 vessel. 



About the time we start to make measurements, a complication be- 

 comes obvious. The calculations above assume ideal conditions. Remem- 

 ber that one arm of the U-tube is open to the atmosphere so that we can 

 compare the pressure inside the vessel with the pressure at the beginning 

 of the experiment, that is, the atmospheric pressure in the room. What 

 happens if the pressure in the room changes? Must we abandon our 

 experiment if a thunderstorm approaches and the barometer falls? No, 

 we simply use an extra manometer containing no living material. This 

 manometer can be used to correct for any changes in atmospheric pres- 

 sure or any slight variations in temperature. This thermobarometer, as it 

 is called, is placed on the water bath along with the experimental 

 manometers. If some variation in the conditions causes a change in the 

 thermobarometer, we simply assume that the other manometers would be 

 affected in the same way. For example, if in one ten-minute interval the 

 thermobarometer rises 2 mm, then each of the other manometers would 

 rise 2 mm in the same time, even without living material. Since the 

 thermobarometer is measuring a change in pressure, it is legitimate to 

 correct the pressure changes of the experimental manometers by adding 

 or subtracting the number of millimeters observed on the thermobarome- 

 ter. Thinking about it for a few seconds tells us in which direction the 

 correction must be applied. 



In an actual series of measurements by the manometric method, the 

 results we obtain will be in millimeters of manometer fluid. The meas- 

 urements would be more useful if they could be expressed as a real 

 amount of a particular gas. "Microliters of oxygen" and "moles of carbon 

 dioxide" have more meaning than "millimeters of manometer fluid." 

 Fortunately, because there is a direct relationship between the pressure 

 change and the change in the amount of gas, we can make the calcula- 

 tions easily. The manometric method is used for small quantities of 

 biological material and measures small quantities of gas. Therefore, the 

 most convenient unit for the gas is the microliter (/Ltl). If we let h repre- 

 sent the number of millimeters of manometer fluid (corrected by the 

 thermobarometer reading) and X the actual amount of gas in microliters, 

 then 



X = kh (10-3) 



or X is directly proportional to h. It can be seen from the gas equation 



