with the
chemical history of combustion, that I must defer all explanation of it
till we come to the examination of that process, which is one of the
most interesting in chemical science.

Light is an agent capable of producing various chemical changes. It is
essential to the welfare both of the animal and vegetable kingdoms; for
men and plants grow pale and sickly if deprived of its salutary
influence. It is likewise remarkable for its property of destroying
colour, which renders it of great consequence in the process of
bleaching.

EMILY.

Is it not singular that light, which in studying optics we were taught
to consider as the source and origin of colours, should have also the
power of destroying them?

CAROLINE.

It is a fact, however, that we every day experience; you know how it
fades the colours of linens and silks.

EMILY.

Certainly. And I recollect that endive is made to grow white instead of
green, by being covered up so as to exclude the light. But by what means
does light produce these effects?

MRS. B.

This I cannot attempt to explain to you until you have obtained a
further knowledge of chemistry. As the chemical properties of light can
be accounted for only in their reference to compound bodies, it would be
useless to detain you any longer on this subject; we may therefore pass
on to the examination of heat, or caloric, with which we are somewhat
better acquainted.

HEAT and LIGHT may be always distinguished by the different sensations
they produce, _Light_ affects the sense of sight; _Caloric_ that of
feeling; the one produces _Vision_, the other the sensation of _Heat_.

Caloric is found to exist in a variety of forms or modifications, and I
think it will be best to consider it under the two following heads, viz.

1. FREE OR RADIANT CALORIC.

2. COMBINED CALORIC.

The first, FREE or RADIANT CALORIC, is also called HEAT OF TEMPERATURE;
it comprehends all heat which is perceptible to the senses, and affects
the thermometer.

EMILY.

You mean such as the heat of the sun, of fire, of candles, of stoves; in
short, of every thing that burns?

MRS. B.

And likewise of things that do not burn, as, for instance, the warmth of
the body; in a word, all heat that is _sensible_, whatever may be its
degree, or the source from which it is derived.

CAROLINE.

What then are the other modifications of caloric? It must be a strange
kind of heat that cannot be perceived by our senses.

MR