Now, to find the mean free path or the average distance between a two collisions, I simply add up all the distances up and divide by the number of collisions. And that's my mean free path. Now, because of this, we can use Grams law to approximate diffusion and we'll see why in a second. So let's look at an illustration of diffusion. Suppose I have a cylindrical tube and I have two claps. Suppose I take this clap, I soak it into ammonia NH three and plug it up.