The engineer’s success in the management of the furnace depends quite
as much upon his handling the air in the right mixtures and proportions
as it does in his using the fuel—for
1. Although invisible to the eye air is as much _a material substance_
as coal or stone. If there were an opening into the interior of the
earth which would permit the air to descend its density would increase
in the same manner at it diminishes in the opposite direction. At the
depth of about 34 miles it would be as dense as water, and at the depth
of 48 miles it would be as dense as quicksilver, and at the depth of
about 50 miles as dense as gold.
2. Air is not only a substance, but _an impenetrable body_; as for
example: if we make a hollow cylinder, smooth and closed at the bottom,
and put a stopper or solid piston to it, no force will enable us to
bring it into contact with the bottom of the cylinder, unless we permit
the air within it to escape.
3. Air is _a fluid_ which is proved by the great movability of its
parts, flowing in all directions in great hurricanes and in gentle
breezes; and also by the fact that a pressure or blow is propagated
through all parts and affects all parts alike.
4. It is also an _elastic fluid_, thus when an inflated bladder is
compressed it immediately restores itself to its former situation;
indeed, since air when compressed restores itself or tends to restore
itself, with the same force as that with which it is compressed, it is
a perfectly elastic body.
5. The weight of a column of air one square foot at the bottom is found
to be 2156 lbs. or very nearly 15 lbs. to the square inch, hence it is
common to state the pressure of the atmosphere as equal to 15 lbs. to
the square inch.
_It follows from these five points that the engineer must consider air
as a positive, although unseen, factor with which his work is to be
accomplished._
What air is composed of is a very important item of knowledge. It
is made of a _mixture_ of two invisible gases whose minute and
inconceivably small atoms are mingled together like a parcel of marbles
and bullets—that is while together they do not lose any of their
distinctive qualities. The two gases are called nitrogen and oxygen,
and of 100 parts or volumes of air 79 parts are of nitrogen and 21
parts of oxygen; but _by weight_ (for the oxygen is the heaviest) 77 of
nitrogen and 23 of oxygen.
The oxygen is the part that furnishes the heat by uniting with the
coal—indeed without it the process of combustion would be impossible:
of the two gases the oxygen is burned in the furnace, more or less
imperfectly, and the nitrogen is wasted.