330 MARKET DAIRYING 
ing particles. When, therefore, heat is applied to the bot- 
tom of a vessel containing water, the particles at the 
bottom do not communicate their heat to the particles next 
above them, but expand and rise, cool ones taking their 
places. This gives rise to convection currents which tend 
to equalize the temperature of the water in the vessel. 
When the water has reached a uniform temperature of 
212° F*. the particles begin to fly off at the surface in the 
form of vapor, and this we call steam. To generate steam 
in a boiler, then, it is necessary to impart to the water in it 
a considerable amount of heat, which is produced by 
burning fuel in the fire box. 
FIRING OF BOILER. 
The immense amount of heat stored in wood and coal 
is rendered effective in the boiler by burning (combus- 
tion). To understand how to fire a boiler intelligently 
one must first learn what the process of burning consists 
of. 
Process of Burning. Anything will burn when the 
temperature has been raised high enough to cause the 
oxygen of the air to unite with it. Thus, in “striking” 
a match the temperature is raised high enough by the 
friction produced to cause the match to burn. The burn- 
ing match will produce heat enough to ignite the kind- 
ling, which in turn, produces the necessary heat to ignite 
the wood or coal in the fire box of the boiler. Burning 
may, therefore, be defined as the union of the oxygen of 
the air with the fuel. In burning a pound of coal or wood 
a definite amount of air must be admitted to furnish the 
necessary oxygen for complete combustion. When oxygen 
is lacking part of the fuel passes out of the chimney un- 
