My question concerns your explanation of why the sky is blue. I have been under the assumption when I discuss this phenomenon with my honors chemistry class that it was not the air (oxygen and nitrogen) gas molecules that scattered the light but rather the dust particles suspended in the atmosphere that was responsible for the scattering. In your suggested demonstration utuilizing powdered milk, is the milk present in the water as molecules or as colloidal suspension particles? When I do a different demonstration I project light from an overhead projector through a one liter beaker of water onto a white screen. The light passes through the pure water unaffected. I then dissolve sodium thiosulfate into the water creating a solution of dissolved ion sized particles. Again the light is unaffected by the presence of atom size pieces of dissolved particles. I then add hydrochloric acid which reacts with the sodium thiosulfate to slowly create colloidal sized supended particles which when enough are produced scatters blue light from the beaker and simultaneously make the image of projected light on the screen turn more yellow and as more suspended particles are formed and more blue light is scattered the projected light turns more orange and then red. So I wonder if it is the atom sized gas particles or the supension sized dust particles that scatter the light. Jeff Green
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