10 BULLETIN 102, UNITED STATES NATIONAL MUSEUM. 
of the molecules is limited only by the volume of the closed contain- 
ing vessel in which they constantly move to and fro. Every mole- 
cule possesses the inherent power of energy and is eternally ener- 
getic within itself. That is, the molecules are in a state of constant 
bombardment against each other and against the sides of the con- 
taining vessel. The most distinguishing characteristic of gas is its 
universal property of completely filling an inclosed space. 
CAUSE OF GAS PRESSURE. 
Gas pressure is the result of the combined efforts of all the moving 
molecules in the gas trying to get farther and farther apart; that is, 
a mass of gas inclosed in a- vessel expands and fills it, and, being 
restrained from further expansion, it exercises a pressure against 
the walls of the vessel. This pressure is the same in all directions on 
equal areas of surface. Not only is every gas molecule eternally 
energetic but its energy may be augumented or retarded by external 
conditions. Contracting the volume of gas increases the intensity of 
its internal molecular motion and therefore increases its pressure. 
Conversely, expanding the volume of a given mass of gas decreases 
the intensity of its internal molecular motion and therefore decreases 
its pressure. That is, with a given mass of gas any increase in volume 
of containing vessel will give the molecules more range of motion and 
thereby lower the pressure. Thus, if a part of a given mass of gas is 
removed from a closed vessel or reservoir the remaining mass of gas 
will expand instanter and keep the vessel or reservoir filled, but at 
a lower pressure. 
WHAT MAKES GAS FLOW. 
The inherent tendency of gas to expand is the basic cause of gas 
flow. Gas flow in pipes or underground reservoirs can not take place 
except between openings of higher, to openings of lower pressure; 
that is, flow can be obtained only by sacrificing pressure. This is in 
accordance with the universal natural law that as long as energy of 
any form undergoes no transformation it tends to gravitate to a 
lower degree of intensity — that is, becomes more stable and ap- 
proaches a universal level of stable equilibrium. Thus, water always 
seeks the lowest level, and confined gas always tends to expand to 
lower pressures. Even where gas compressors are used to increase 
the pressure by contracing the volume, the gas is not pushed through 
the pipe like a plug of incompressible fluid, like oil or water, but 
goes through by virtue of the increased expansive force resulting 
from the higher pressure. 
DEFINITION OF NATURAL GAS. 
Natural gas is a highly combustible gas made by a secret process 
of nature. It is not a chemical compound — as*popularly supposed — 
