Diffusion of bromine gas. Bromine liquid in the bottom glass cylinder is evaporating, producing an orange gas. When allowed into contact with the cylinder filled with air (top), the two tend to try to equalise by molecules moving from an area of higher concentration to one of lower - upward for the bromine and downward for the air. In this way it is seen that diffusion a process that does not involve the bulk movement of a gas (as opposed to simple mixing).