If we look at the kinetic theory expressions for the viscosity, thermal conductivity, and diffusivity, we can express them in the following form:

The product of velocity and mean free path has the dimensions of length \({ }^{2} /\) time, or diffusivity. In the above equation, all quantities on the left-hand side represent a diffusivity, specifically the same diffusivity. Comment on what this means for the mechanism whereby mass, energy, and momentum are transported in gas. Would you expect the same type of mechanism to operate in a liquid? In a solid? What would be required for the same mechanism to operate?