156 NOVUM ORGANUM 



the extremities will grow cold in that rod first where it is 

 placed beneath, as the contrary takes place with regard to 

 heat. 



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

 form expansive motion of the whole, but of the small par 

 ticles of the body; and this motion being at the same time 

 restrained, repulsed, and reflected, becomes alternating, per 

 petually 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 de 

 tached 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 im 

 pediment or repulsion, the heat is not perceptible. In con 

 fined 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. 



