122 NOVUM ORGANUM. 



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, there 

 fore, heating is excluded from the form of heat. Even when 

 heat is excited by the approach 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 multipli 

 cation, 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 mo 

 tion, 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 forward 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 exuda 

 tion 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 

 melted or dissolved, are however softened. The same cir 

 cumstance takes place in sticks of wood, which become 

 flexible when a little heated in warm ashes. 



It is most readily observed in air, which instantly and 

 manifestly expands with a small degree of heat, as in Inst. 

 38, Tab. 3. 



It is also shown in the contrary nature of cold. For cold 

 contracts and narrows every substance ; so that in intense 

 frosts nails fall out of the wall and brass cracks, and heated 

 glass exposed suddenly to the cold cracks and breaks. So 

 the air by a slight degree of cold contracts itself, as in 



