326 NOVUM ORGANUM. 



body is destroyed, or at least remarkably altered, by all strong and 

 vehement fire and heat. Whence it is quite clear that heat causes 

 tumult and perturbation, and brisk motion in the internal parts of the 

 body, which perceptibly tends to its dissolution. 



But when we say of motion that it stands in the place of a genus 

 to heat, we mean to convey not that heat generates motion, or motion, 

 heat (although even both may be true in some cases), but that essential 

 heat, or the &quot;quid ipsum &quot; of heat, is Motion and nothing else; 

 limited, however, by Differences, which we shall presently subjoin, 

 when we have added some cautions for the avoiding of ambiguity. 



Heat, as regards the senses, is a relative thing, and bears relation 

 to man, and not to the universe, and is rightly defined as merely the 

 effect of heat on animal spirit ; moreover, it is in itself a variable 

 thing, for the same body (as the senses are predisposed) induces a 

 perception both of heat and cold, as is clear from Instance 41, 

 Table 3. 



Nor indeed ought the communication of heat, or its transitive 

 Nature, by which a body grows hot when applied to a hot body, to be 

 confounded with the Form of Heat, for heat is one thing and heating 

 another. Heat is induced by motion of attrition, without any pre 

 ceding heat ; whence heating is excluded from the Form of Heat. 

 And even when heat is produced by the approximation of a hot body, 

 this is not the result of the Form of Heat, but depends altogether on 

 a higher and more common Nature, viz. on the Nature of assimilation 

 or self-multiplication ; a subject into which a separate inquiry must be 

 made. 



Again, our notion of fire is popular and worthless ; for it is made up 

 of the combination of heat and light in any body, as in common flame 

 and bodies heated to redness. 



And so, all ambiguity being removed, we must at length come to 

 true Differences, which limit motion, and constitute it the Form of 

 Heat. 



The First Difference then is this ; that heat is an expansive motion, 

 by which a body strives to dilate and to betake itself into a larger 

 sphere or dimension than it previously occupied. This Difference is 

 most strongly displayed in flame, when smoke or thick vapour mani 

 festly dilates and opens itself into flame. 



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

 and bubble, and continue the process of self-expansion until they are 

 changed into a far more extended and diluted body than the liquid 

 itself, viz. in vapour, smoke, or air. 



It is shown also in all wood and combustible matter, where exuda 

 tion takes place sometimes, but evaporation always. 



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

 texture) do not easily swell and dilate themselves ; but yet their spirit, 

 being itself dilated, and so imbibing a desire for further dilation, 

 forces and drives the grossest parts into a liquid state. But if the heat 

 be much intensified, it dissolves and changes much of their substance 

 into vapour. 



