480 NOVUM ORGANUM. [ROOK II, 



III. The third difference is this ; that heat is not a uniform 

 expansive motion of the whole, but of the small particles of the 

 body ; and this motion being at the same time restrained, re- 

 pulsed, and reflected, becomes alternating, perpetually hurrying, 

 striving, struggling, and irritated by the repercussion, which is 

 the source of the violence of flame and heat. 



But this difference is chiefly shown in flame and boiling liquids, 

 which always hurry, swell, and subside again in detached parts. 



It is also shown in bodies of such hard texture as not to swell 

 or dilate in bulk, such as red-hot iron, in which the heat is most 

 violent. 



It is also shown by the fires burning most briskly in the 

 coldest weather. 



It is also shown by this, that when the air is dilated in the 

 thermometer uniformly and equably, without any impediment or 

 repulsion, the heat is not perceptible. In confined draughts also, 

 although they break out very violently, no remarkable heat is 

 perceived, because the motion affects the whole, without any 

 alternating motion in the particles ; for which reason try whether 

 flame do not burn more at the sides than in its centre. 



It is also shown in this, that all burning proceeds by the 

 minute pores of bodies, undermining, penetrating, piercing, 

 and pricking them as if with an infinite number of needle-points. 

 Hence all strong acids (if adapted to the body on which they 

 act) exhibit the effects of fire, from their corroding and pungent 

 nature. 



The difference of which we now speak is common also to the 

 nature of cold, in which the contracting motion is restrained by 

 the resistance of expansion, as in heat the expansive motion is 

 restrained by the resistance of contraction. 



Whether, therefore, the particles of matter penetrate inwards 

 or outwards, the reasoning is the same, though the power be very 

 different, because we have nothing on earth which is intensely cold. 



I\ r . The fourth difference is a modification of the preceding, 

 namely, that this stimulating or penetrating motion should be 

 rapid and never sluggish, and should take place not in the very 

 minutest particles, but rather in those of some tolerable dimen 

 sions. 



It is shown by comparing the effects of fire with those of 

 time. Time dries, consumes, undermines, and reduces to ashes 

 as well as fire, and perhaps to a much finer degree ; but as its 

 motion ia very slow, and attacks very minute particles, no heat 

 is perceived. 



It is also shown in a comparison of the dissolution of iron and 

 gold ; for gold is dissolved without the excitement of any heat, 

 but iron with a vehement excitement of it, although most in the 

 *ame time, because in the former the penetration of the svpv* 



