from which most that is practical in future domestic life is wholly
excluded. We therefore give a brief outline of some of the leading
scientific principles which every housekeeper should understand and
employ, in order to perform successfully one of her most important
duties.

Concerning the essential nature of heat, and its intimate relations
with the other great natural forces, light, electricity, etc., we shall
not attempt to treat, but shall, for practical purposes, assume it to
be a separate and independent force. Heat or caloric, then, has certain
powers or principles. Let us consider them:

First, we find _Conduction_, by which heat passes from one particle
to another next to it; as when one end of a poker is warmed by placing
the other end in the fire. The bodies which allow this power free
course are called conductors, and those which do not are named
non-conductors, Metals are good conductors; feathers, wool, and furs
are poor conductors; and water, air, and gases are non-conductors.

Another principle of heat is _Convection_, by which water, air, and
gases are warmed. This is, literally, the process of _conveying_ heat
from one portion of a fluid body to another by currents resulting from
changes of temperature. It is secured by bringing one portion of a
liquid or gas into contact with a heated surface, whereby it becomes
lighter and expanded in volume. In consequence, the cooler and heavier
particles above pressing downward, the lighter ones rise upward, when
the former, being heated, rise in their turn, and give place to others
again descending from above. Thus a constant motion of currents and
interchange of particles is produced until, as in a vessel of water, the
whole body comes to an equal temperature. Air is heated in the same way.