NOVUM ORGANUM 393 



arrow or dart, for instance, has both a rotatory and progressive 

 motion. In the same way the motion of heat is both expansive 

 and tending upwards. 



This difference is shown by putting the tongs or poker into 

 the fire. If placed perpendicularly with the hand above, they 

 soon burn it, but much less speedily if the hand hold them 

 sloping or from below. 



It is also conspicuous in distillations per desccnsum, which 

 men are wont to employ with delicate flowers, whose scent 

 easily evaporates. Their industry has devised placing the fire 

 above instead of below, that it may scorch less ; for not only 

 flame but all heat has an upward tendency. 



Let an experiment be made on the contrary nature of cold, 

 whether its contraction be downwards, as the expansion of heat 

 is upwards. Take, therefore, two iron rods or two glass tubes, 

 alike in other respects, and warm them a little, and place a 

 sponge, dipped in cold water, or some snow, below the one and 

 above the other. We are of opinion that 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 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 hurry- 

 ing, 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 impediment 

 or repulsion, the heat is not perceptible. In confined drafts 

 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 effect of fire, from their corroding and pungent 

 nature. 



