Dust

Dust is present in the atmosphere from various causes. The existence of dust in the air of a room or in a liquid is proved by the fact that we can trace the course of a beam of light through the air or the liquid by means of its partial reflection from suspended moles. The dust in the atmosphere is carried up in part from the earth's surface by currents of air, and in part by evaporation. A great part of it is due to volcanic action, and part also, the so-called 'cosmic dust,' has a meteoric origin (see METEORS). The blue colour of the sky is due to the reflection of light from particles of dust, possibly even from particles of air. Aitken has shown that no condensation of moisture in the air (as in rain, mist, fog; see FOG) could occur without nuclei such as dust particles. He has also devised an apparatus for counting the number of dust particles in a given sample of air or gas.