Number of particles is given by `n=-D(n_(2)-n_(1))/(x_(2)-x_(1))` crossing a unit area perpendicular to `X`-axis in unit time, where `n_(1)` and `n_(2)` are number of particles per unit volume for the value of `x` meant to `x_(2)` and `x_(1)`. Find dimensions of `D` called as diffusion constant