ISO HISTORY OF THER3IOTICS. 



amount of labor, of persevering and combined observation, the progress 

 of this branch of knowledge requires. I do not even speak of the con- 

 dition of the more elevated parts of the atmosphere. The diminution 

 of temperature as we ascend, one of the most marked of atmospheric 

 facts, has been variously explained by different writers. Thus Dalton 2 * 

 (1808) refers it to a principle "that each atom of air, in the same per- 

 pendicular column, is possessed of the same degree of heat," which 

 principle he conceives to be entirely empirical in this case. Fourier 

 says 29 (1817), " This phenomenon results from several causes : one of 

 the principal is the progressive extinction of the rays of heat in the 

 successive strata of the atmosphere." 



Leaving, therefore, the application of thermotical and atmological 

 principles in particular cases, let us consider for a moment the general 

 views to which they have led philosophers. 



CHAPTER IV. 



PHYSICAL THEORIES OF HEAT. 



IIEX 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 w r as presented to us by physical astronomy, optics, and 

 acoustics. In these sciences, the niaintainers of a distinct and compre- 

 hensive theory have professed at least to show that it explains and in- 

 cludes the principal laws of phenomena of various kinds ; in Thermo- 

 tics, 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 refrac- 

 tion ; 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 vet exhibit credentials of this kind. 



* New Syst. of Chcm. vol. i. p. 125. " Ann. Chim. vi. 285. 



