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The basic definition of the density of a substance is the ratio of the mass of a given amount of the substance to the volume it occupies. For liquids, this definition is generally satisfactory. However, since gases are compressible, further clarification is required.

The mean density is the ratio of the mass of a given amount of a substance Mean density to the volume that this amount occupies. If the mean density in all parts of a substance is the same then the density is said to be uniform

he density at a point is the limit to which the mean density tends as the Density at a point volume considered is indefinitely reduced, that is limv→0(m/V). As a math- ematical definition this is satisfactory; since, however, all matter actually consists of separate molecules, we should think of the volume reduced not absolutely to zero, but to an exceedingly small amount that is nevertheless large enough to contain a considerable number of molecules. The concept of a continuum is thus implicit in the definition of density at a point

The relative density is the ratio of the density of a substance to some standard Relative density density. The standard density chosen for comparison with the density of a solid or a liquid is invariably that of water at 4◦C. For a gas, the standard density may be that of air or that of hydrogen, although for gases the term is little used. (The termspecific gravity has also been used for the relative density of a solid or a liquid, but relative density is much to be preferred.) As relative density is the ratio of two magnitudes of the same kind it is merely a numeric without unit

Now that I have bored you to fucking tears.......

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