e unless there be a space above the
water. But the expanding water has stored up the heat which would have
raised steam, and the moment expansion begins after fracture this energy
is suddenly let loose. Steam forms instantaneously, augmenting the
effects of the explosion. From this it will be gathered that all pipes
should be properly protected against frost; especially near the roof.

Another cause of disaster is the _furring up_ of the pipes with the lime
deposited by hard water when heated. When hard water is used, the pipes
will sooner or later be blocked near the boiler; and as the deposit is
too hard to be scraped away, periodical renewals are unavoidable.

HOW A LAMP WORKS.

From heating we turn to lighting, and first to the ordinary paraffin
lamp. The two chief things to notice about this are the wick and the
chimney. The wick, being made of closely-woven cotton, draws up the oil
by what is known as _capillary attraction_. If you dip the ends of two
glass tubes, one half an inch, the other one-eighth of an inch in
diameter, into a vessel of water, you will notice that the water rises
higher in the smaller tube. Or get two clean glass plates and lay them
face to face, touching at one end, but kept slightly apart at the other
by some small object. If they are partly submerged perpendicularly, the
water will rise between the plates--furthest on the side at which the
two plates touch, and less and less as the other edge is approached. The
tendency of liquids to rise through porous bodies is a phenomenon for
which we cannot account.

Mineral oil contains a large proportion of carbon and hydrogen; it is
therefore termed hydro-carbon. When oil reaches the top of a lighted
wick, the liquid is heated until it turns into gas. The carbon and
hydrogen unite with the oxygen of the air. Some particles of the carbon
apparently do not combine at once, and as they pass through the fiery
zone of the flame are heated to such a temperature as to become highly
luminous. It is to produce these light-rays that we use a lamp, and to
burn our oil efficiently we must supply the flame with plenty of oxygen,
with more than it could naturally obtain. So we surround it with a
transparent chimney of special glass. The air inside the chimney is
heated, and rises; fresh air rushes in at the bottom, and is also heated
and replaced. As the air passes through, the flame seizes on the oxygen.
If the wick is turned up until the flame becomes sm