232 FRIEDLANDER AND PASCERI Cartwright were obtained with a thermal precipitator. The shapes of the spectra obtained in this study and in those by Junge and Cartwright are similar above a radius of 0.04 y, This is consistent with the theory of self-preserving size distributions proposed by Friedlander,!!)!2)4 Below 0.04 uthe shape of the distribution is subject to variation with time and location. It should be noted that the distributions reported in this study are time-averaged over both one-half day and one day; Junge’s data are the average of several determinations taken over short (a few minutes) consecutive intervals and Cartwright’s distributions represent single determinations over equally short sampling times. CONCLUSIONS This new sampling method has a numberof important advantages, It is very Simple, and it provides a large surface over which deposition is uniform. The relation of the distribution of the deposited sample to the distribution in the gas is known from theory. Efficiency of removal increases as particle size decreases, The method should be easily adaptable to autoradiographic techniques. REFERENCES 1. R. E, Pasceri, The Size Distribution of Atmospheric Aerosols, Ph.D. Dissertation, Johns Hopkins University, Baltimore, Md., 1964. 2. V. G. Levich, Physicochemical Hydrodynamics, pp. 65-72, Prentice-Hall, Inc., Englewood Cliffs, N. J., 1962. 3. E. M. Sparrow and J. L. Gregg, Heat Transfer from a Rotating Disk to Fluids of any Prandtl] Number, J. Heat Transfer, 81: 249-251 (1959). 4, M. Litt and G. Serad, Chemical Reactions on a Rotating Disk, Part II— Experimental, to be published in Chemical Engineering and Science. 5. D. R. Olander, Unsteady-state Heat and Mass Transfer in the Rotating Disk System, Intern. J. Heat Mass Transfer, 5: 825-836 (1962). 6. D. W. Jordan, The Adhesion of Dust Particles, Brit. J. Appl. Phys., 5 (Suppl. 3): S194-S197 (1954). 7. C, E, Hall, Measurement of Globular Protein Molecules by Electron Microscopy, J. Biophys. Biochem. Cytol., 7: 613-618 (1960). 8. C. N. Davies, Definitive Equations for the Fluid Resistance of Spheres, Proc. Phys. Soc., 57: 259-270 (1945). 9. C, Junge, The Size Distribution and Aging of Natural Aerosols as Deter- minedfrom Electrical and Optical Data onthe Atmosphere, J. Meteorol., 12: 13 (1955). 10. J. Cartwright, The Electron Microscopy of Airborne Dusts, Brit. J. Appl. Phys., 5 (Suppl. 3): 8109-S119 (1954). 11. S. K. Friedlander, On the Particle Size Spectrum of Atmospheric Aerosols, J. Meteorol., 17: 373 (1960). 12, 8. K,. Friedlander, Similarity Considerations for the Particle-Size Spectrum of a Coagulating, Sedimenting Aerosol, J. Meteorol., 17: 479 (1960). 13. D. L. Swift and S, K. Friedlander, The Coagulation of Hydrosols by Brownian Motion and Laminar Shear Flow, J. Coll. Sci.,19: 621 (1964).