When the molecules in any region of a fluid medium possess an excess of energy, concentration, or momentum, such that gradients of these properties exist in the neighborhood of this region, the system will attempt to restore spatial uniformity by transporting the relevant property in the direction of the deficient region. The transport occurs even in the absence of any bulk motion in this direction. As an illustration, consider a body of stagnant gas situated between two parallel plates as shown in Figure 4.1.1a. If plate A is suddenly raised to a temperature TA > TB, then the region around plate B also will be heated soon. Therefore there can be a net transfer of heat from A to B despite the lack of any bulk fluid movement. Similarly if there is initially a higher concentration of a species at A relative to its concentration at B, then its molecules will slowly migrate from A to B, as shown in Figure 4.1.1b. Finally, if plate A is impulsively started to move in a direction parallel to itself (Figure 4.1.1c), then plane B will soon feel the motion and, if unrestrained, will tend to be dragged along. Note that in the last case even though there is a bulk flow in the direction parallel to the plates, the transport occurs in the direction normal to the plates in which there is no motion.