milk last.

CAROLINE.

That is a natural consequence of their different bulks; the lead being
the smallest body, will be heated soonest, and the milk, which is the
largest, will require the longest time.

MRS. B.

That explanation will not do, for if the lead be the least in bulk, it
offers also the least surface to the caloric, the quantity of heat
therefore which can enter into it in the same space of time is
proportionally smaller.

EMILY.

Why, then, do not the three bodies attain the temperature of the oven at
the same time?

MRS. B.

It is supposed to be on account of the different capacity of these
bodies for caloric.

CAROLINE.

What do you mean by the capacity of a body for caloric?

MRS. B.

I mean a certain disposition of bodies to require more or less caloric
for raising their temperature to any degree of heat. Perhaps the fact
may be thus explained:

Let us put as many marbles into this glass as it will contain, and pour
some sand over them--observe how the sand penetrates and lodges between
them. We shall now fill another glass with pebbles of various forms--you
see that they arrange themselves in a more compact manner than the
marbles, which, being globular, can touch each other by a single point
only. The pebbles, therefore, will not admit so much sand between them;
and consequently one of these glasses will necessarily contain more sand
than the other, though both of them be equally full.

CAROLINE.

This I understand perfectly. The marbles and the pebbles represent two
bodies of different kinds, and the sand the caloric contained in them;
it appears very plain, from this comparison, that one body may admit of
more caloric between its particles than another.

MRS. B.

You can no longer be surprised, therefore, that bodies of a different
capacity for caloric should require different proportions of that fluid
to raise their temperatures equally.

EMILY.

But I do not conceive why the body that contains the most caloric should
not be of the highest temperature; that is to say, feel hot in
proportion to the quantity of caloric it contains?

MRS. B.

The caloric that is employed in filling the capacity of a body, is not
free caloric; but is imprisoned as it were in the body, and is therefore
imperceptible: for we can feel only the caloric which the body parts
with, and not that which it retains.

CAROLINE.

It appears to me very extraordinary that heat should be confined in