Density
A number of techniques as well as standards exist for the measurement of density of materials. Such techniques include the use of a hydrometer (a buoyancy method for liquids), Hydrostatic balance (a buoyancy method for liquids and solids), immersed body method (a buoyancy method for liquids), pycnometer (liquids and solids), air comparison pycnometer (solids), oscillating densitometer (liquids), as well as pour and tap (solids). However, each individual method or technique measures different types of density (e.g. bulk density, skeletal density, etc.), and therefore it is necessary to have an understanding of the type of density being measured as well as the type of material in question.
Viscosity
where formula_58 is the Boltzmann constant, formula_59 the molecular mass, and formula_60 a numerical constant on the order of formula_61. The quantity formula_62, the mean free path, measures the average distance a molecule travels between collisions. Even without "a priori" knowledge of formula_60, this expression has interesting implications. In particular, since formula_62 is typically inversely proportional to density and increases with temperature, formula_10 itself should increase with temperature and be "independent" of density at fixed temperature. In fact, both of these predictions persist in more sophisticated treatments, and accurately describe experimental observations. Note that this behavior runs counter to common intuition regarding liquids, for which viscosity typically "decreases" with temperature.