475 sovuM ORGAKUM. [book [1 



Wliat we have said with regard to motion must be thus under- 

 stood, when taken as the genus of heat : it must not be thought 

 that heat generates motion, or motion heat (though in some 

 respects tliis be true), but that the very essence of heat, or the 

 substantial self of heat, is motion and nothing else, limited, 

 how^ever, by certain differences which we will presently add, after 

 giving some cautions for avoiding ambiguity. 



Sensible heat is relative, and regards man, not universe ; and 

 is rightly held to be merely the effect of heat on animal spirit. 

 It is even variable in itself, since the same body (in different 

 states of sensation) excites the feeling of heat and of cold ; this 

 is shown by Inst. 41, Tab. 3. 



IS^or should we confound the communication of heat or its 

 transitive nature, by which a body grows warm at the approach 

 of a heated body, with the form of heat ; for heat is one thing 

 and heating another. Heat can be excited by friction without 

 any previous heating body, and, therefore, heating is excluded 

 from the form of heat. Even when heat is excited by the ap- 

 proach of a hot body, this depends not on the form of heat, but 

 on another more profound and common nature ; namely, that of 

 assimilation and multiplication, about which a separate inquiry 

 must be made. 



The notion of fire is vulgar, and of no assistance ; it is merely 

 compounded of the conjunction of heat and light in any body, as 

 in ordinary flame and red-hot substances. 



Laying aside all ambiguity, therefore, we must lastly consider 

 the true differences which limit motion and render it the form of 

 heat. 



I. The first difference is, that heat is an expansive motion, by 

 which the body strives to dilate itself, and to occupy a greater 

 space than before. This difference is principally seen in flame, 

 where the smoke or thick vapour is clearly dilated and bursts 

 into flame. 



It is also shown in all boiling liquids, which swell, rise, and 

 boil up to the sight, and the process of expansion is urged for- 

 ward till they are converted into a much more extended and 

 dilated body than the liquid itself, such as steam, smoke, or air. 



It is also shown in wood and combustibles where exudation 

 sometimes takes place, and evaporation always. 



It is also shown in the melting of metals, which, being very 

 compact, do not easily swell and dilate, but yet their spirit, when 

 dilated and desirous of further expansion, forces and urges its 

 thicker parts into dissolution, and if the heat be pushed still 

 farther, reduces a considerable part of them into a volatile state. 



It is also shown in iron or stones, which, though not mcU<jdi or 



* "Quid ipsum," the to tI i)v dyai of Aristotle., 



