This is accomplished by reason of the differences in the boiling points of water and alcohol.
These standards are the freezing and boiling points of distilled water.
So with the boiling points of liquids; they rise as atmospheric pressure increases, they fall as atmospheric pressure is reduced.
Since most liquids have their boiling points reduced about 40 C. by operating in vacuo, the advantage of the vacuum is apparent.
Thus 0° and 100° on the centigrade scale are respectively the freezing and the boiling points of water under normal pressure.
This is taken as the standard pressure in determining the volumes of gases as well as the boiling points of liquids.
The various hydrocarbons distill over in the general order of their boiling points.