The basic feature of a fluid is that it can flow, and this is the essence of any definition of it. This feature, however, applies to substances that are not true fluids, e. g. a fine powder piled on a sloping surface will also flow. Fine powder, such as flour, poured in a column on to a flat surface will form a roughly conical pile, with a large angle of repose, whereas water, which is a true fluid, poured on to a fully wetted surface will spread uniformly over the whole surface. Equally, a powder may be heaped in a spoon or bowl, whereas a liquid will always form a level surface. A definition of a fluid must allow for these facts. Thus a fluid may be defined as ‘matter capable of flowing, and either finding its own level (if a liquid), or filling the whole of its container (if a gas)’.

Experiment shows that an extremely fine powder, in which the particles are not much larger than molecular size, will also find its own level and may thus come under the common definition of a liquid. Also a phenomenon well known in the transport of sands, gravels, etc. is that they will find their own level if they are agitated by vibration, or the passage of air jets through the particles. These, however, are special cases and do not detract from the authority of the definition of a fluid as a substance that flows or (tautologically) that possesses fluidity.

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