An increase in pressure causes a gas to contract.

A decrease in pressure causes a gas to expand.

An increase in temperature causes a gas to expand.

A decrease in temperature causes a gas to contract.

Specific gravity or relative weight of a gas is the weight of a gas compared to an equal volume of normal air under the same temperature and pressure.

The specific gravity of normal air is 1.0.

A gas that is heavier than air has a specific gravity higher than 1.0.

A gas that is lighter than air has a specific gravity less than 1.0.

Gases issuing into still air without mixing tend to stratify according to the gas s specific gravity.

Light gases diffuse rapidly and are easy to disperse.

Heavy gases diffuse slowly and are difficult to disperse.

A gas that will burn is "flammable."

Flammable gases can explode under certain conditions.

The range of concentrations within which a gas will explode is known as its "explosive range."

Solubility is the ability of a gas to dissolve in water.

Stirring, pumping or wading through water can release large amounts of soluble gas into the mine atmosphere.

Certain gases can be toxic (poisonous) or asphyxiants (suffocants).

Normal Air: SG 1, Colorless, odorless, tasteless.  78% nitrogen, 21% oxygen, 1% argon, carbon dioxide, neon, helium, krypton, xenon, hydrogen, methane, nitrous oxide, ozone.

Oxygen: SG 1.1054.  Soluble.  Nonflammable.  21% of normal air.  19.5% legal requirement.  17% = panting.  15% = dizziness & headache.  9% = unconsciousness.

Carbon Monoxide: SG 0.9672.  Slightly soluble.  Colorless, odorless, tasteless.  Toxic.  TLV = 50ppm MSHA, 35ppm OSHA.