The human nose is very sensitive to certain molecules. For example, it can sense the presence of the chemical CH₃SH (methyl mercaptan) at levels as small as 2 parts per billion. Another especially smelly substance is ammonia, NH₃. A student in a lab with very still air has two vials placed 20 cm from his nose; one contains methyl mercaptan, the other ammonia. He begins his experiment and removes the caps from both vials, and after 30 minutes, he notices the smell of ammonia. How soon after does his nose detect the methyl mercaptan? Is diffusion a good model for how fast odors travel?