Mechanical mixture due to pulmonary flow, in contrast to mixing by molecular diffusion within the respiratory spaces, has been studied directly using aerosol inhalation. The technique uses particles, about ½ μ in diameter, which are fairly stable in the respiratory tract, and measures the concentration of aerosol during expiration with a continuous and rapid detector of scattered light and, simultaneously, the expiratory volume flow. The flow component of pulmonary ventilation is described in terms of the fractions of tidal air (Vx) and functional residual air (Vr) which participate in the ventilatory exchange of aerosol. Aerosol clearance experiments on three apparently healthy subjects show these two newly defined mixing volumes (Vx and Vr) to be much smaller than their analogous volumes measured by gas clearance: less than one-third for the fraction of tidal air and less than one-tenth for the fraction of functional residual air. These results demonstrate quantitatively the major role of molecular diffusion in ventilation. This approach provides a new method for the measurement of the mechanical component of intrapulmonary mixing. Submitted on November 3, 1958
The EPR linevridth spectrum of triplet excitons in single-crystal tetracene at room temperature has been measured at 24 6Hz and explained by the theory presented in the preceding paper. The existence of a residual width attributable to orientational disorder has been incorporated in the analysis. The hopping rate derived from these results is shown to be at odds~ith a recent measurement of diffusion in the tetracene lattice.
A simple, direct, and inexpensive determination of the speed of sound in air, suitable for an undergraduate laboratory exercise, is presented. The same apparatus is used to demonstrate acoustic pulse reflections from closed and open ended tubes of circular cross section.
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