Imágenes de páginas
PDF
EPUB

grow shrivelled; so likewise the kernels of nuts contract and shrink from their shells.

Old cheeses have their coats wrinkled; posts, wood-pillars, pales, &c. contract in their dimensions by long standing, crack, gape and separate; especially when set up green. And the like happens in solid bowls or balls of wood.

The earth cracks and chops in great droughts, and becomes full of chinks on its surface; and these cracks sometimes reach so deep, that water issues out thereat.

ADMONITION.

Let no one triflingly pretend, that this contraction in the case of dryness, is no more than a consumption of humidity; for if that were all, and only the moisture, converted into spirit, flew off; bodies would then retain their former dimensions, and barely become cavernous, like cork or pumice; and not be locally contracted, and lessened in their dimensions*.

HISTORY.

Clay is burnt into brick and tile in the kiln; but if the heat be violent, as in the middle of the furnace, some part of the clay is also changed in its nature, and runs into glass.

* See Dr. Hook's Micrographia passim ; and Dr. Grew's Anatomy of Plants.

If wood be set on fire, and the flame be stifled, the wood turns to a coal or a substance more light and spongy than wood*.

Most of the metals when covered in a crucible, and set in the fire, especially in a reverberatory furnace, are converted into a friable substance, and calcined.

Many fossil, or metalline, and some vegetable matters, are vitrified by a strong fire.

All bodies capable of tumefaction turn to a coaly substance, and contract their dimensions, if too long exposed to the fire.

Paper, parchment, skins, &c. are not only made to wrinkle in their parts by the fire; but also to curl, or coil, and wind their whole substance into a roll.

Linen cloth being set on flame, aud presently extinguished, turns into a light substance, which scarce takes flame again, but easily ignites; as in the case of tinder.

Fat bodies, as wax, butter, oil, &c. become scorched, foul, and as it were smoky by the fire.

Eggs cóntract in bulk by the fire, and change the transparency of their whites into an opake whiteness.

If an egg be broke into high rectified spirit of

+For the full process, see Mr. Evelyn's Sylva; or Boerhaave's Chemistry.

wine, it turns white and hard; as if it were heated over the fire. So likewise bread steeped in the same spirit, appears almost as if it had been toasted.

OBSERVATIONS.

1. So long as the spirit is detained in a body, and excited and dilated by fire, or heat; so long it keeps itself in agitation, endeavours its own escape, softens, supples, and fuses the tangible parts together and thus to digest, subdue, and work together the parts of bodies, is the proper office of the spirit. But after the spirit has once found an exit, and is discharged, then the work of the parts takes place; and, having been tortured by the spirit, now combine together, and wedge themselves close; as well through an appetite of connexion and mutual contract, as an aversion to motion and disturbance. And upon this follow closeness, hardness, and a stubborness of the body.

2. There is a limit and ultimate end of the process of the contraction in the parts of bodies by fire; for if the quantity of matter be too small to cohere through the violent depredation of the fire, the parts desert each other, are turned to ashes and calcined. And thus much for the contractions occasioned by the discharge of the spirit of bodies; whether it proceed from age, fire, or potential heat.

SECT. XV.

OF THE CONTRACTIONS OF BODIES BY ACTUAL,

EXTERNAL COLD.

TRANSITION.

RECIPROCAL to the action of dilatation by actual external heat, is the action of contraction by actual external cold. And this condensation is, of all others, the most genuine and proper; and would likewise, be the most powerful, if we had here, upon the surface of the earth, any intense degree of cold. But cold, or a remission of heat, (both which we here consider together) simply condenses some things, without altering their nature; restores others, though imperfectly, that are rarified; and again, perfectly converts and transforms others, by condensation, from one nature to another: each of which we must here touch upon, in their turn.

HISTORY.

1. Air, in a thermometer, is sensible of the degrees both of cold and heat. And, in the winter; we sometimes placed a kind of cap of snow, upon the head of the glass; which has so much increased the cold, even in a snowy season, as to raise the water a few degrees higher, by condensing the air*.

*See Novum Organum, part ii. aph. 13.

2. We above observed, that the air in this glass was dilated a third; and contracted itself as much upon a remission of the heat.

PRECEPT.

1. It deserves to be tried by particular experiments, whether air dilated with heat, might be fixed in its expansion; so as not to endeavour its own restoration and contraction. Take therefore, a strong glass tube, heat it violently, then perfectly close up the orifice, that the air may not contract; and let the glass stand for some days thus closed: afterwards plunge it, still stopt, into water; open it under the water, and observe how much liquor it draws in; or how the quantity is, in proportion to what it would have attracted, if the glass had been directly put into water.*

2. Observe likewise, by the way, with regard to the title of heat and cold, whether air so strongly dilated, and forcibly detained, retains its heat much longer than when the orifice of the glass is left open.

HISTORY.

1. In very clear and cold nights, during the winter, the stars appear larger than in the se

The experiment might be better tried in the condenser with strongly compressed air. See Mr. Boyle's Experiments upon the Spring of the Air.

« AnteriorContinuar »