244 OBSERVATIONS ON THE NATURE OF HEAT. 
susceptible of change, it cannot upset the idea that caloric is ma- 
terial. 
The radiation of caloric is a sufficient proof, were there no other, 
that it is material. It passes through space in straight lines; it can 
be reflected at will, and collected into a point, so as to resemble the 
condensation of bodies confessedly material. It is searcely possible 
to conceive that this would arise merely from the commotion of the 
particles of 2 body, for the hypothesis implies the greater the com- 
motion the greater is the quantity of caloric; and there is no evi- 
dence that the commotion of the particles upon which it is concen~ 
trated is greater than those from which it is reflected, unless the 
increase of temperature be taken as such; besides, the point from 
which the caloric emanates, and the point upon which it is eoneen- 
trated, may be under nearly similar circumstances with regard to this 
agent, yet they must be governed by very different laws: the former 
being increased in temperature from the commotion of its particles ; 
while, in the latter, the increase ef temperature causes the commo- 
tion among the particles, or, in other words, what is the cause in 
one point is the effect in the other. 
How ill adapted, then, must the hypothesis be that requires the aid 
of opposite laws to explain the same phenomena in the same matter ! 
The like reasoning is applicable to the conduction or the communica- 
tion of caloric from one body to another; but with the idea that ca- 
Toric is material, the phenomena of radiation and conduction admit of 
easy explanation. 
The hypothesis, also, of its material nature, gives a satisfactory 
explanation of the phenomena attendant on the conversion of solid 
substances into fluids or gases, or the reverse: in the former, caloric 
is absorbed ; in the latter, it is given off. How great is the resem- 
blance between this and the absorption of a fluid by a sponge or any 
porous body, and the escape of that fluid on pressure. 
There are, however, some instances in nature, conirary to the ge- 
neral rule, where caloric is produced on the conversion of a body 
from a smaller to a larger bulk. Of this, the explosion of gunpow- 
der is an example, and which is brought forward against the idea of 
the material nature of caloric. But the fact proves nothing, save by 
analogy ; and if analogy have any weight, there is as much reason to 
suspect that caloric, as matter, is lodged among the particles of this 
astonishing product, ready to burst forth on the application of a suit- 
able cause. 
A similar objection may be raised from the fact that water, on 
oe 
