224 FRIEDLANDER AND PASCERI air. They measured the rates of dissolution of benzoic acid, cinnamic acid, and 8-naphthol in water and the rateof evaporation of naphthalene in air. Theexperimental results agreed well with theory. The rotating disk acts as a kind of centrifugal fan drawing in air from afar and hurling it out at its periphery. Continuity requires that the component of the fluid velocity normal to the disk be finite even at an infinite distance from the disk surface. The limiting value of this velocity component is 0.886 (w)v)”; the limiting value is reached to a close approximation not far from the disk surface at a distance about equal to 5 (v/wy)*. For w) = 2000 rpm, the experimental condition, and vy=0,150 cm’/sec for air, the limiting velocity of 5 cm/sec was attained at about 1.5 mm from the disk surface, With reasonable precautions against major drafts, velocities of this magnitude will be of controlling importance in the transfer process. Olander’ has considered the response of the transport rate toa rotating disk to a sudden concentration change at its surface. A step change of concentration at the surface is equivalent to a step change at the edge of the concentration boundary layer. For Schmidt numbers between 10? and 10*, Olander found the mass transfer rate was within 5% of the new steady rate in the times t = 8/w, and t = 30/wo, respec- tively. For the experimental wy used in this study, the maximum time was about 0.25 sec. Since the response time of the disk is so small, it may be considered to follow changes of concentration in the ambient atmosphere instantaneously. Hence, if a size-distribution function is determined for a sample run, it will have the time-averaged form n(r) _ fin(r,t) at = where 7 is the sampling time. Consistent with the theory of the diffusion battery, it is assumed that the disk is a perfect sink; i.e., Cy = 0. Since a particle adhering to a rotating disk is subjected to centrifugal force, it is necessary to consider the magnitude of the force at the experimental rotation speeds. In this work, traversing in the electron microscope was limited to an inner circle of 0.03 cm in radius on the sampling grid. The acceleration on a particle situated on the circumference of this circle is about 2 G. Jordan® has presented calculations on the approximate forces required to dislodge small particles from surfaces and has shownthat they are considerably greater than 2G.