A study is made of the aspiration of dust, suspended in an ideal fluid, into a bulky sampling head with an orifice in the top. The behaviour of the particles is characterized by the value of a parameter k which is proportional to the square of the particle size divided by the rate of suction. When k is greater than 2, gravity governs the movement of the particles and when it is less than 1/(3 + 2√2) the influence of suction predominates. For values exceeding 1/(3 + 2√2) a shadow free from dust extends to infinity below the head and particles settle on the top. For smaller values of k there is no deposition on the head and the shadow area is finite. An indication is given of the way in which the system could be employed for particle size analysis by selective sampling.