BOOK II.] AfHoaisMS. 479 



dissolved, are Iiowever softened. The aame circumstance 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 mani- 

 festly expands with a small detrree of'heat, as 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 ^lass 

 exposed suddenly to the cold cracks and breaks. So the air by a 

 slight dcjjree 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 differences, 

 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 

 dilating motion, but cold a contracting and eondensing motion. 



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

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

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

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

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

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

 and tendinir 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 tho hand hold them sloping or 

 from below. 



It is also conspicuous in distillations per descensum, 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. "VVe 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. 



■» To show the error of the text, we need only mention the case of 

 water, which, when confined in corked vases, and exposed to the action 

 of a freezing Jitmosphere, is sure to swell out and break tiiose vessel* 

 "srhioh are not sufficiently large to contain its expanded volume. Megv 

 iotti narrates a hundred other instances of a similar character. Ed. 



