Density

Comparative weights of solids and liquids may be expressed by either density or specific gravity. Density is defined as the weight per unit volume. Comparative densities of gases are expressed by specific volumes. Specific volume is the volume of one pound of a gas at standard conditions. Standard conditions are considered to be 68 f. and 29.92 inches of mercury column pressure.

The volume of one pound of air at standard conditions is 12.388 cubic feet. As a comparison, one pound of hydrogen occupies 178.9 cubic feet and one pound of ammonia (r-717) occupies 21 cubic feet, while carbon dioxide (r-744) only occupies 8.15 cubic feet. If one pound of a gas occupies a greater space than air, we think of it as being a light gas while if it occupies less space than air it is classified as a heavy gas.