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CHAPTER IV. 

 PHYSICAL THEORIES OF HEAT. 



WHEN we look at the condition of that branch 

 of knowledge which, according to the phras- 

 eology already employed, we must call Physical 

 Thermotics, in opposition to Formal Thermotics, 

 which gives us detached lav s 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 pro- 

 fessed at least to show that it explains and includes 

 the principal laws of phenomena of very various 

 kinds : in thermotics, we have only attempts to 

 explain a part of the facts. We have here no ex- 

 ample 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. 



