A dasymeter is a densimeter used to measure the density of gases. It consists of a thin sphere made of glass that is immersed in the gas and weighed. A hydrometer is an instrument used for determining the specific gravity of liquids. ... In physics, density is defined as mass m per unit volume V. Mathematically, it is expressed as where, in SI units: ρ (rho) is the density of the substance, measured in kg·m-3 m is the mass of the substance, measured in kg V is the volume of the substance... A gas is one of the four major phases of matter (after solid and liquid, and followed by plasma, that subsequently appear as a solid material is subjected to increasingly higher temperatures. ... A sphere is a perfectly symmetrical geometrical object. ... Glass can be made transparent and flat, or into other shapes and colors as shown in this sphere from the Verrerie of Brehat in Brittany. ... In the physical sciences, weight is a measurement of the gravitational force acting on an object. ...