CHAPTER IV.
Physical Theories of Heat.
WHEN we look at the condition of that branch of knowledge which, according to the phraseology already employed, we must call Physical Thermotics, in opposition to Formal Thermotics, which gives us detached laws of phenomena, we find the prospect very different from that which was presented to us by physical astronomy, optics, and acoustics. In these sciences, the maintainers of a distinct and comprehensive theory have professed at least to show that it explains and includes the principal laws of phenomena of various kinds; in Thermotics, we have only attempts to explain a part of the facts. We have here no example of an hypothesis which, assumed in order to explain one class of phenomena, has been found also to account exactly for another; as when central forces led to the precession of the equinoxes, or when the explanation of polarization explained also double refraction; or when the pressure of the atmosphere, as measured by the barometer, gave the true velocity of sound. Such coincidences, or consiliences, as I have elsewhere called them, are the test of truth; and thermotical theories cannot yet exhibit credentials of this kind. [181]
On looking back at our view of this science, it will be seen that it may be distinguished into two parts; the Doctrines of Conduction and Radiation, which we call Thermotics proper; and the Doctrines respecting the relation of Heat, Airs, and Moisture, which we have termed Atmology. These two subjects differ in their bearing on our hypothetical views.
Thermotical Theories.—The phenomena of radiant heat, like those of radiant light, obviously admit of general explanation in two different ways;—by the emission of material particles, or by the propagation of undulations. Both these opinions have found supporters. Probably most persons, in adopting Prevost’s theory of exchanges, conceive the radiation of heat to be the radiation of matter. The undulation hypothesis, on the other hand, appears to be suggested by the production of heat by friction, and was accordingly maintained by Rumford and others. Leslie[68] appears, in a great part of his Inquiry, to be a supporter of some undulatory doctrine, but it is extremely difficult to make out what his undulating medium is; or rather, his opinions wavered during his progress. In page 31, he asks, “What is this calorific and frigorific fluid? and after keeping the reader in suspense for a moment, he replies,
“Quod petis hic est.
It is merely the ambient AIR.” But at page 150, he again asks the question, and, at page 188, he answers, “It is the same subtile matter that, according to its different modes of existence, constitutes either heat or light.” A person thus vacillating between two opinions, one of which is palpably false, and the other laden with exceeding difficulties which he does not even attempt to remove, had little right to protest against[69] “the sportive freaks of some intangible aura;” to rank all other hypotheses than his own with the “occult qualities of the schools;” and to class the “prejudices” of his opponents with the tenets of those who maintained the fuga vacui in opposition to Torricelli. It is worth while noticing this kind of rhetoric, in order to observe, that it may be used just as easily on the wrong side as on the right.
[68] An Experimental Inquiry into the Nature and Propagation of Heat, 1804.
[69] Ib. p. 47.
Till recently, the theory of material heat, and of its propagation by emission, was probably the one most in favor with those who had studied mathematical thermotics. As we have [said], the laws of [182] conduction, in their ultimate analytical form, were almost identical with the laws of motion of fluids. Fourier’s principle also, that the radiation of heat takes place from points below the surface, and is intercepted by the superficial particles, appears to favor the notion of material emission.