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Arthur B. DuBois is first author of the 1956 primary report introducing a rapid plethysmographic measurement of thoracic gas volume and comparing it with a nitrogen-washout measurement of functional residual capacity in normal subjects.^[ref] The experiment turns a gas-compression relation into an observable pressure-volume slope.

The apparatus enclosed the subject in a sealed chamber, used a mouthpiece shutter and recorded chamber and mouth-pressure signals with capacitance manometers on a cathode-ray oscillograph. Chamber calibration related its pressure signal to changes in body volume.^[ref] This supplied simultaneous signals rather than an estimate from one uncalibrated pressure reading.

An original Boyle's law example follows the paper's derivation. For an approximately isothermal closed gas compartment,Here $\Delta V$ is the signed change in gas volume, not a raw chamber-pressure signal. If $|\Delta P|\ll P_0$, the approximation is $V_0\approx -P_0\Delta V/\Delta P$. The original article derives the same exact-to-small-change distinction.^[ref]

For stipulated $P_0=100$ kPa, compression $\Delta V=-0.030$ L and pressure rise $\Delta P=1$ kPa, the exact expression gives 3.03 L and the small-change approximation gives 3.00 L. Their 0.03 L difference is an approximation effect in the chosen gas model, not observed instrument error in the study.

The experiment also depends on the relationship between mouth and alveolar pressure. The paper explicitly discusses failures when that relationship is disrupted.^[ref] DuBois's contribution combines the physical law, pressure sensing, chamber calibration and tests of the assumptions needed to interpret the signal.

Body plethysmography uses pressure and volume changes in an enclosing chamber to measure respiratory mechanics and compressible thoracic gas volume. For lung-volume measurement, a brief closed-airway effort relates gas compression inside the lungs to the chamber signal. The gas volume belongs to the instant of occlusion; it is not automatically residual lung volume.^[ref]

In the original 1956 experiment, Arthur B. DuBois and colleagues used an airtight chamber of approximately 600 L, a mouthpiece shutter, capacitance pressure transducers and a cathode-ray oscillograph. One signal represented mouth pressure and the other chamber pressure, converted to volume change through instrument calibration.^[ref] An open communication between mouth and alveolar gas during the closed-airway effort is essential to interpreting the mouth signal.

For an ideal isothermal gas amount, Boyle's law givesWith stipulated effective absolute gas pressure $P=100$ kPa, compression $\Delta V=-0.006$ L and increase $\Delta P=0.2$ kPa, the exact expression gives 3.006 L. Neglecting the product of small changes gives the first-order result $V\simeq -P\Delta V/\Delta P=3.0$ L. Here $\Delta V$ is the inferred lung-gas change, not an unconverted box-pressure reading.

If the measurement starts at quiet end-expiration, the interpreted lung-volume reference is functional residual capacity; linked inspiratory capacity and vital capacity maneuvers then provide other capacities. The thermal behavior of gas, instrument response and mouth-to-alveolar pressure equilibration remain physical assumptions to assess. A precise algebraic fit cannot repair a pressure signal that represents a different compartment.

Unlike tracer dilution, compression can probe gas that is not freely reached by the tracer during the test. This explains why body plethysmography and nitrogen washout can give different results without being duplicate names for the same observable.^[ref]

Residual lung volume, RV, is the gas remaining in the lungs after maximal expiration. It is smaller than the quiet end-expiratory functional residual capacity by the expiratory reserve volume:Thus residual gas is not the tidal volume or a direct measurement of gas exchanged in one breath.^[ref]

For stipulated FRC 2.5 L and expiratory reserve volume 1.1 L, RV is 1.4 L. Since spirometry records changes rather than an absolute lung-gas baseline, exhaling into a spirometer cannot by itself identify the gas still inside.

The 1956 experiment by Arthur B. DuBois and colleagues made that hidden volume accessible through body plethysmography. Their apparatus combined an airtight chamber, a mouthpiece shutter, capacitance pressure transducers and an oscillograph. Respiratory efforts against the closed shutter related mouth-pressure changes to changes in chamber volume.^[ref]

The physical idea follows Boyle's law. For a fixed gas amount at approximately fixed temperature, writeTo first order in the small changes,An original ideal calculation uses effective gas pressure 100 kPa, pressure increase 0.2 kPa and volume decrease 4 mL, giving $V\simeq 2000$ mL. This estimates the gas volume at the instant of airway closure. If closure occurs at quiet end-expiration, the result corresponds to an FRC-level measurement; expiratory reserve volume must still be subtracted to obtain residual lung volume. Pressure equilibration, thermal assumptions and correct volume conventions are part of the physical measurement, not consequences of the algebra alone.