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 interstices are by heat lessened, and would by extreme heat be entirely obliterated.
CAROLINE.