12 
BULLETIN 102, UNITED STATES: NATIONAL MUSEUM. 
vapor in the presence of its own liquid. A gas is the form which 
any liquid assumes above its critical temperature, and it can not be 
liquefied by pressure alone, but only by its combined pressure and 
cooling. All vapors are gases, but not all gases are vapors. The dif- 
ference between vapors and gases may be summarized as 1 follows: 
Aeriform fluids. 
Vapor. Gas. 
Below its critical temperature and Above its critical temperature, 
pressure. Can be condensed only by both pres- 
Can be condensed by pressure alone, sure and cold. 
Gasoline found in natural gas always exists there in the form of a 
vapor, while methane, for instance, in natural gas exists only as 
a gas. 
NATURAL GAS* MAY BE WET OR DRY. 
Natural gases coming from the ground may be classed — according 
to their gasoline vapor content — into two main groups, namely : 
1. Wet gas . — This is gas intimately associated with oil, usually 
produced with oil, and is ordinarily known as casing head natural 
gas. 
2. Dry gas . — This is gas not intimately associated with oil, but 
may nevertheless contain gasoline vapors. The term “ dry ” does 
not refer to water vapor that may be carried by the gas, but rather 
to the gasoline vapor, and, furthermore, this is a relative term since 
a strictly dry gas would be one containing no gasoline vapors. 1 
WRONG IMPRESSION OF WORD “ NATURAL.” 
While natural gas is a natural product made by nature, it is no 
more natural than other minerals, like coal, oil, or iron ore. The 
word “ natural ” came into common use probably as contrasted with 
manufactured gas, and the use of the word appears to have given a 
fallacious impression that natural gas was a free and unlimited 
resource. Merely being made by nature does not mean that a sub- 
stance is cheap and of low value. Natural gas is a natural resource, 
which men have learned to use for the satisfaction of their wants. 
The misconception regarding the position of natural gas has arisen 
from failing to appreciate that man creates no new matter and can 
merely get the materials of nature ready for consumption. Food, 
clothing, wealth in all its forms, are derived 'originally from nature. 
The forces of nature, working through the ages, have created things 
which mankind needs. Human effort expended on these products of 
nature, converts them into forms which are usable. 2 
1 For further discussion, see Bureau of Mines Bulletins No. 88 and No. 120. 
3 Suggested by Ely’s Outlines of Economics. 
