ot as boiling water; but we do
not know the positive quantity of heat contained either in freezing or
boiling water, any more than we know the real extremes of heat and cold;
and consequently we cannot determine that of the body in which the
thermometer is placed.

CAROLINE.

I do not quite understand this explanation.

MRS. B.

Let us compare a thermometer to a well, in which the water rises to
different heights, according as it is more or less supplied by the
spring which feeds it: if the depth of the well is unfathomable, it must
be impossible to know the absolute quantity of water it contains; yet we
can with the greatest accuracy measure the number of feet the water has
risen or fallen in the well at any time, and consequently know the
precise quantity of its increase or diminution, without having the least
knowledge of the whole quantity of water it contains.

CAROLINE.

Now I comprehend it very well; nothing appears to me to explain a thing
so clearly as a comparison.

EMILY.

But will thermometers bear any degree of heat?

MRS. B.

No; for if the temperature were much above the highest degree marked on
the scale of the thermometer, the mercury would burst the tube in an
attempt to ascend. And at any rate, no thermometer can be applied to
temperatures higher than the boiling point of the liquid used in its
construction, for the steam, on the liquid beginning to boil, would
burst the tube. In furnaces, or whenever any very high temperature is to
be measured, a pyrometer, invented by Wedgwood, is used for that
purpose. It is made of a certain composition of baked clay, which has
the peculiar property of contracting by heat, so that the degree of
contraction of this substance indicates the temperature to which it has
been exposed.

EMILY.

But is it possible for a body to contract by heat? I thought that heat
dilated all bodies whatever.

MRS. B.

This is not an exception to the rule. You must recollect that the bulk
of the clay is not compared, whilst hot, with that which it has when
cold; but it is from the change which the clay has undergone by _having
been_ heated that the indications of this instrument are derived. This
change consists in a beginning fusion which tends to unite the particles
of clay more closely, thus rendering it less pervious or spongy.

Clay is to be considered as a spongy body, having many interstices or
pores, from its having contained water when soft. These interstic