OF NATURAL PHILOSOPHY. 313 



busiion, by which a portion of carbon, an inflam- 

 mable principle existing in the blood, is united with 

 the oxygen of the air in respiration ; and thus car- 

 ried off from the system : fermentation is nothing 

 more than a decomposition of chemical elements 

 loosely united, and their re-union in a more per- 

 manent state of combination. The analogy between 

 the sun and terrestrial fire is so natural as to have 

 been chosen by Newton to exemplify the irresistible 

 force of an inference derived from that principle. 

 But the nature of the sun and the mode in which 

 its wonderful supply of light and heat is maintained 

 are involved in a mystery which every discovery that 

 has been made either in chemistry or optics, so far 

 from elucidating, seems only to render more pro- 

 found. Friction as a source of heat is well known : 

 we rub our hands to warm them, and we grease 

 the axles of carriage-wheels to prevent their setting 

 fire to the wood ; an accident which, in spite of 

 this precaution, does sometimes happen. But the 

 effect of friction, as a means of producing heat with 

 little or no consumption of materials, was not fully 

 understood till made the subject of direct expe- 

 riment by count Rumford, whose results appear to 

 have established the extraordinary fact, that an 

 unlimited supply of heat may be derived by friction 

 from the same materials. Condensation, whether 

 of air by pressure, or of metals by percussion, is 

 another powerful source of heat. Thus, iron may 

 be so dexterously hammered as to become red-hot, 

 and the rapid condensation of a confined portion of 

 air will set tinder on fire. 

 (348.) The most violent heats known are pro- 



