BOOtt II.] THE NATURE OF HEAT 481 



rating acid is mild, and gently insinuates itself, and the particles 

 of gold yield easily, but the penetration of iron is violent, and 

 attended with some struggle, and its particles are more ob- 

 stinate. 



It is partially shown, also, in some gangrenes and mortifica- 

 tions of flesh, which do not excite great heat or pain, from the 

 gentle nature of the putrefaction. 



Let this suffice for a first vintage, or the commencement of 

 the interpretation of the form of heat by the liberty of the un- 

 derstanding. 



From this first vintage the form or true definition of heat 

 (considered relatively to the universe and not to the sense) is 

 briefly thus: — Heat is an expansive motion restrained, and 

 striving to exert itself in the smaller particles." The expansion 

 is modified by its tendency to rise, though expanding towards 

 the exterior; and the effort is modified by its not being sluggish, 

 but active and somewhat violent. 



With regard to the operative definition, the matter is the same. 

 If you are able to excite a dilating or expansive motion in any 

 natural body, and so to repress that motion and force it on itself 

 &i not to allow the expansion to proceed equally, but only to be 

 partially exerted and partially repressed, you will beyond all 

 doubt produce heat, without any consideration as to whether the 

 body be of earth (or elementary, as they term it), or imbued with 

 celestial influence, luminous or opaque, rare or dense, locally 

 expanded or contained within the bounds of its first dimensions, 

 verging to dissolution or remaining fixed, animal, vegetable, or 



" Bacon's inquisition into the nature of heat, as an example of the 

 mode of interpreting nature, cannot be looked upon otherwise than as a 

 complete failure. Though the exact nature of this phenomenon is still an 

 obscure and controverted matter, the science of thermotics now con- 

 sists of many important truths, and to none ot these truths is there so 

 much as an approximation in Bacon's process. The steps by which this 

 science really advanced were the discoveiy of a measure of a heat 

 or temperature, the establishment of the laws ot conduction and 

 radiation, of the laws ot specific heat, latent heat, and the like. Such 

 advances have led to Ampere's hypothesis, that heat consists in the 

 vibrations of an imponderable fluid ; and to Laplace's theory, that tem- 

 perature consists in the internal radiation of a similar medium. These 

 hypotheses cannot yet be said to be even probable, but at least they are 

 BO modified as to include some of the preceding laws which are firmly 

 established, whereas Bacon's ** form," or true definition of heat, as 

 stated in the text, includes no laws of phenomena, explains no process, 

 and is indeed itself an example of illicit generalization. 



In all the details of his example of heat he is unfortunate. He m- 

 tludes in his collection of instances, the hot tastes of aromatic plants, 

 the caustic effects of acids, and many other facts which cannot b« 

 ftscribed to heat without a studious laxity in the use of the word. £d» 

 a 2i 



