Fick's second law is a fundamental equation in the study of stable diffusion. It describes the rate at which a substance diffuses through a medium over time. Mathematically, Fick's second law is represented as:

In this equation, C represents the concentration of the diffusing substance at a given location and time, D is the diffusion coefficient, t denotes time, and x is the spatial coordinate. The negative sign indicates that the diffusion occurs in the direction of decreasing concentration.

Fick's second law is significant as it establishes a quantitative relationship between the concentration gradient and the diffusion process. It allows us to predict how the concentration of a substance changes over time and space, providing valuable insights in various scientific disciplines.