NATURAL, GAS. 
11 
but a mechanical mixture of several combustible and diluent gases 
and vapors thoroughly diffused through each other, the number and 
exact proportion of the various crude natural constituents varying 
for the different localities and somewhat during the working lives 
of individual wells. 
The term “ casing-head gas ” is applied to a natural gas that Sows 
from oil wells, coming out between the casing and tubing. It is col- 
lected by means of a metal head — called “ braden-head ” 1 — connect- 
ing the casing with the tubing, as shown by the dotted lines at the 
top of fig. 1. The term “braden-head gas” is sometimes used 
synonymously for casing-head gas. 
DEFINITION OF “ MECHANICAL MIXTURE.” 
This is a mixture where two or more substances are brought 
together in a thoroughly commingled state, without, however, any of 
the constituent substances losing their individual identity. The 
various vapors and gases going to make up natural gas are merely 
intermingled as mechanical mixtures. Another very common illus- 
tration is atmospheric air, where water vapor and the gases oxygen 
and nitrogen are merely mixed in the form of a mechanical mixture ; 
that is, the water vapor has undergone no chemical change and the 
oxygen and nitrogen have undergone no chemical change by the 
mixture. 
DEFINITION OF TERM “ VAPOR.” 
This word literally means a warm exhalation. A vapor is the 
gaseous state of a substance which at ordinary temperature exists 
as a solid or liquid; that is, the vapor is the result of the action 
of heat on a solid or liquid. On removal of the heat the vapor will 
return to its former solid or liquid state. When a liquid, by the 
action of heat, goes into a vapor or gaseous form it is said to vaporize 
or evaporate, the meaning of these two terms being the same. The 
most common form of vapor is the moisture always present in greater 
or less degree in the atmospheric air. 
GASES AND VAPORS DISTINGUISHED. 
A vapor is an aeriform substance in the gaseous state at any tem- 
perature below the critical point, the critical point being the line 
of demarcation between a vapor and a gas. The temperature of 
fluid at the critical point is the critical temperature, and the pressure 
which at this critical temperature just suffices to condense the gas 
to the liquid form is called the critical pressure. A vapor can be re- 
duced to a liquid by pressure alone, and may exist as a saturated 
1 Named after its designer Mr. Glenn T. Braden. 
