For 10 points, name this quantity equal to mass over volume.
The Kepler conjecture states that this quantity is maximized for HCP and FCC crystals. The reciprocal of this quantity is called specific volume. This quantity is the ratio between kinematic and dynamic viscosity. The melting point line has a negative slope when this quantity is greater for liquids than for solids. The ratio of it to a reference standard is specific gravity. Among elements, this quantity is greatest for osmium and iridium. For water, it's maximized at 4 degrees Celsius. If two substances are immiscible, the one with a greater value for this will form the bottom layer. For 10 points, name this quantity equal to mass over volume.
The Kepler conjecture states that this quantity is maximized for HCP and FCC crystals.
density [or packing efficiency before "volume"]