e very little caloric in return for what is absorbed, the room
would necessarily be cooled by it.

MRS. B.

Just so; and as in consequence of the mirrors, a more considerable
exchange of rays takes place between the ice and the thermometer, than
between these and any of the surrounding bodies, the temperature of the
thermometer must be more lowered than that of any other adjacent object.

CAROLINE.

I confess I do not perfectly understand your explanation.

MRS. B.

This experiment is exactly similar to that made with the heated bullet:
for, if we consider the thermometer as the hot body (which it certainly
is in comparison to the ice), you may then easily understand that it is
by the loss of the calorific rays which the thermometer sends to the
ice, and not by any cold rays received from it, that the fall of the
mercury is occasioned: for the ice, far from emitting rays of cold,
sends forth rays of caloric, which diminish the loss sustained by the
thermometer.

Let us say, for instance, that the radiation of the thermometer towards
the ice is equal to 20, and that of the ice towards the thermometer to
10: the exchange in favour of the ice is as 20 is to 10, or the
thermometer absolutely loses 10, whilst the ice gains 10.

CAROLINE.

But if the ice actually sends rays of caloric to the thermometer, must
not the latter fall still lower when the ice is removed?

MRS. B.

No; for the space that the ice occupied, admits rays from all the
surrounding bodies to pass through it; and those being of the same
temperature as the thermometer, will not affect it, because as much heat
now returns to the thermometer as radiates from it.

CAROLINE.

I must confess that you have explained this in so satisfactory a manner,
that I cannot help being convinced now that cold has no real claim to
the rank of a positive being.

MRS. B.

Before I conclude the subject of radiation I must observe to you that
different bodies, (or rather surfaces,) possess the power of radiating
caloric in very different degrees.

Some very curious experiments have been made by Mr. Leslie on this
subject, and it was for this purpose that he invented the differential
thermometer; with its assistance he ascertained that black surfaces
radiate most, glass next, and polished surfaces the least of all.

EMILY.

Supposing these surfaces, of course, to be all of the same temperature.

MRS. B.

Undoubtedly. I will now show you the very simple a