240 OBSERVATIONS ON THE NATURE OF HEAT. 
which we are acquainted, unless acting through very Jong periods 
of time. 
Concerning the very important practical question of the extension 
of the coal-measures beneath the new red sandstone districts, I am 
not at present prepared to offer any thing farther than was stated in 
the last number, except that some facts I met with tended to confirm 
me in the opinion of the present boundaries of the coal-fields, when 
ending abruptly against the new red sandstone, having been formed 
by denuding and eroding forces acting before the deposition of that 
rock, rather than by direct fractures and dislocations having marked 
them out, either before or since. If this opinion should be correct, 
the existence of coal measures beneath any part of the new red 
sandstone can only be determined by direct experiment, since we 
have no means of inferring to what depth eroding forces may have 
acted. It is, at all events, a circumstance well worthy of cautious 
examination before entering into an expensive undertaking in search 
of coal beyond the present fields. 
OBSERVATIONS ON THE NATURE OF HEAT. 
Tue word heat, as used in common language, expresses a cause 
and its effect ; it expresses the sensation of heat and the cause of that 
sensation: hence philosophers, to avoid looseness of speech, have 
determined to strip the word of its two-fold meaning, and to confine 
it to the sensation, while, for the cause, they have framed a new 
word, viz. caloric. This distinction, I conceive, will appear suffi- 
ciently important to adopt it in the following remarks. 
When the attention is first drawn to this subject, it may possibly 
be thought an easy matter to determine the nature of a principle so 
universal as caloric; but that men of the greatest fame in science 
differ in opinion upon its nature, will be ample refutation of the sim- 
plicity of the question. At present there prevail two opinions: the 
one is, that caloric is a subtle fluid, capable of entering into bodies 
and of being emitted from them; the other, that it is merely caused 
by the motion excited among the particles of matter; or, in other 
words, the one holds that caloric is material, while the other, that it 
is merely a property of matter. In entering upon this inquiry, it 
a 
