BACON 



of assimilation and multiplication, about which a separate in- 

 quiry 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 vapor 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 

 further, 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 circumstance 

 takes place in sticks of wood, which become flexible when a lit- 

 tle heated in warm ashes. 



It is most readily observed in air, which instantly and mani- 

 festly expands with a small degree of heat, as stated in Inst. 38, 

 Tab. 3. 



It is also shown in the contrary nature of cold ; for cold con- 

 tracts and narrows every substance; so that in intense frosts 

 nails fall out of the wall and brass cracks, and heated glass ex- 

 posed suddenly to the cold cracks and breaks. So the air by 

 a slight degree of cold, contracts itself, as in Inst. 38, Tab. 3. 

 More will be said of this in the inquiry into cold. 



Nor is it to be wondered at if cold and heat exhibit many com- 

 mon effects (for which see Inst. 32, Tab. 2), since two differ- 

 ences, of which we shall presently speak, belong to each nature : 

 although in the present difference the effects be diametrically 

 opposed to each other. For heat occasions an expansive and di- 

 lating motion, but cold a contracting and condensing motion. 



II. The second difference is a modification of the preceding, 

 namely, that heat is an expansive motion, tending towards the 

 exterior, but at the same time bearing the body upwards. For 

 there is no doubt that there be many compound motions, as an 



