mmediate nature, that we must direct our attention.

The connection between light and heat is very obvious; indeed, it is
such, that it is extremely difficult to examine the one independently of
the other.

EMILY.

But, is it possible to separate light from heat; I thought they were
only different degrees of the same thing, fire?

MRS. B.

I told you that fire was not now considered as a simple element. Whether
light and heat be altogether different agents, or not, I cannot pretend
to decide; but, in many cases, light may be separated from heat. The
first discovery of this was made by a celebrated Swedish chemist,
Scheele. Another very striking illustration of the separation of heat
and light was long after pointed out by Dr. Herschell. This philosopher
discovered that these two agents were emitted in the rays of the sun,
and that heat was less refrangible than light; for, in separating the
different coloured rays of light by a prism (as we did some time ago),
he found that the greatest heat was beyond the spectrum, at a little
distance from the red rays, which, you may recollect, are the least
refrangible.

EMILY.

I should like to try that experiment.

MRS. B.

It is by no means an easy one: the heat of a ray of light, refracted by
a prism, is so small, that it requires a very delicate thermometer to
distinguish the difference of the degree of heat within and without the
spectrum. For in this experiment the heat is not totally separated from
the light, each coloured ray retaining a certain portion of it, though
the greatest part is not sufficiently refracted to fall within the
spectrum.

EMILY.

I suppose, then, that those coloured rays which are the least
refrangible, retain the greatest quantity of heat?

MRS. B.

They do so.

EMILY.

Though I no longer doubt that light and heat can be separated, Dr.
Herschell's experiment does not appear to me to afford sufficient proof
that they are essentially different; for light, which you call a simple
body, may likewise be divided into the various coloured rays.

MRS. B.

No doubt there must be some difference in the various coloured rays.
Even their chemical powers are different. The blue rays, for instance,
have the greatest effect in separating oxygen from bodies, as was found
by Scheele; and there exist also, as Dr. Wollaston has shown, rays more
refrangible than the blue, which produce the same chemical effect, and,
what is very remarkable, are