when you walk by a lady wearing perfume one and you can smell it as you walk by but do we really notice wat all is going on first there is the factor of latent heat or evaporation and this is do to the fact from the heat you can feel and the heat generated by the body wearing the perfume the next is diffusion. diffusion works by taking small particals in the air and dispercess it threw the air and that is why u can smell the perfume from a couple of feet away of the smell leangers for a while after she has left.