Diffusion from spherical cavity

Although the basic features of tiiis process are the same as those of the spherical cavity (Fickian diffusion through a variable area), we use it to cast a somewhat wider net by considering a composite cylinder made up of different materials witii different dijfusivities. This leads us to the concept of resistances in series, and the principle that flows from it, the additivity of resistances. Botii the concept and the principle are so pervasive in mass (and heat) transfer tirat tiiey deserve an early introduction. 

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