16 BULLETIN 102, UNITED STATES NATIONAL MUSEUM. 
actual expansion of gas in lowering the pressure from 150 pounds 
down to less than 10 pounds would be about 6 per cent greater than 
that given by literal application of the law. This has the practical 
effect of making leakage in main lines and natural gas distributing 
plants sometimes seem considerably less than it actually is, due to 
failure to recognize that in expanding from high pressure to low 
the gas actually increases in volume more than the exact literal ap- 
plication of the law would give. 1 
EFFECT OF TEMPERATURE ON GAS VOLUME, KNOWN AS CHARLES’ LAW. 
“ The volume of a given mass of any gas under constant pressure 
increases from the freezing point by a constant fraction of its volume 
at zero.” This starts from the absolute zero of the gas, which is 
492° F. below freezing, as shown at the right. In other words, the 
gas will expand 1/492 of its volume at 32° F. for each degree 
F ahrenheit rise of temperature. 
This makes the change in volume directly proportional to the 
absolute temperature and means that approximately each 5° F. 
increase in temperatures makes an increase of 1 per cent in volume 
and each 5° F. decrease in temperature makes a decrease of 1 per 
cent in volume. For specific application of this see page 471. 
GAS SAND OR GAS ROCK. 
In no case is the gas found in rooms, caverns, or large crevices, 
as popularly supposed. “The oil and gas sands are simply very 
porous rocks which contain not only one great cavity, but millions 
upon millions of small or microscopic cavities, so that the oil, gas, 
water, or all three together, it may be, occupy these numerous little 
spaces, and thus saturate the rock just as water does a piece of cloth 
or a sponge when dipped into the same. The larger these pores are, 
and the greater the volume they occupy in proportion to the volume of 
the rock mass, the greater will be the contained oil or gas supply, 
and this proportion in fairly good producing sands, usually varies 
between one-fifth and one-tenth.” 2 
DEFINITION OF ROCK PRESSURE. 
When nature generated or deposited the natural gas in the rock 
reservoir — made up of the microscopic cavities between the sand 
grains — a fixed amount of gas was placed in a fixed inclosed space. 
The pressure in the rock — called “ rock pressure ” — was the result 
of the pressing into this fixed rock space of a larger volume of gas 
than the mere free air capacitjr of this rock reservoir. The degree 
of compression employed by nature in the formation process deter- 
1 Deviation of Natural Gas from Bojde’s Law, by Robert F. Earhart and Samuel S. 
Wyer. Transactions American Society Mechanical Engineers, vol. 38, p. 285. 
2 1. C. White, West Virginia Geological Survey, vol. 1, p. 155. 
