4072. Air

4072. Air. Tho air or atmosphere which surrounds tho earth is a mixture (not. combination - Fownes) of 77 parts by weight

(or 79.19 parts by measure) of nitrogen, and 23 parts by weight (or 20.81 by measure) of oxygen. It usually contains also a variable amount of moisture, a very small proportion of carbonic acid, a trace of ammonia, and sometimes of carburetted hydrogen; these last are found incidentally in the air, in a variable degree. It is the standard in the comparative or specific gravity of gaseous bodies. (See No. 47.) At 60° Fahr., and with the barometer at 30 inches, 100 cubic inches of air weigh 30.935 grains; and water (the standard of specific gravity for fluids) weighs just 816 times as much as air.

4073. Tests for Pure Air

4073. Tests for Pure Air. A simple method of ascertaining the presence of impurity (carbonic acid) in the atmosphere, is to nearly fill a glass tumbler with lime water, and to place it in any convenient position, as on the mantelpiece of a room. The rapidity with which a pellicle forms on its surface, or the water becomes cloudy, corresponds to the amount of the carbonic acid present in the atmosphere that surrounds it.

A little moist carbonate of lead put on a plate or saucer, and exposed in the same way, will turn black, should any sulphuretted hydrogen be contained in the air. This is a very delicate test for that destructive gas.