Biltz (Ber., 1888, 21, p. 2013; 1901, 34, p. 2490) showed that the vapour density decreased with the temperature, and also depended on the pressure.
This subject owes its importance in modern chemistry to the fact that the vapour density, when hydrogen is taken as the standard, gives perfectly definite information as to the molecular condition of the compound, since twice the vapour density equals the molecular weight of the compound.
For higher temperatures the bulb of the vapour density tube is made of porcelain or platinum, and is heated in a gas furnace.
It has the exact vapour-density corresponding to the formula.
This fact, coupled with the determination of the vapour density of the gas, establishes the molecular formula CO.
