Of Attraction and Electricity.

77

nefs; this is only a comparative Term, and implies no more than the Difference of the Weight of Bodies compared together, or their specific Gravity.

A. Pray, what Distinction do you make between Gravity and Attraction?

B. In the Nature of the Thing there is no Difference, they are both one and the fame Principle, only in the Manner of confidering it this Diftinction arifeth; if we refpect it in the Body, containing the Center of Gravity, we call this Power, in that Body, Attraction; but we call it Gravity in Bodies which are moved toward that Center, or Body, wherein it is: Thus the Earth is faid to attract Bodies on it, or falling on it; and thofe Bodies are faid to gravitate towards the Earth; fo the Loadftone attracts Steel, and the Steel gravitates to it.

A. Is this Power of Attraction the fame in all Bodies, of the fame Kind, of equal Denfity and Magnitude?

B. Yes; but in all fuch Bodies, the less any one is than another, the greater is its attracting Force; fo the magnetic Attraction is ftronger in a small Load-ftone, in Proportion to its Weight, than in a larger

one.

A. Pray, what is the Confequence of this?

B. Sir

B. Sir Ifaac Newton has on this Score found, by Calculation, that, as the Particles of Light are the leaft Bodies we know of, fo their Attraction is 1000ooooooooooog Million of Millions of Times greater than that of the Earth, on the Earth's Surface; according to the Quantity of Matter in each, and the afore-mentioned Velocity of Light.

A. Pray, Sir, what do you understand by Electricity?

B. A certain Kind of attractive Faculty peculiar to fome Bodies, as Amber, Jet, Sealing-wax, Glass, &c, whofe Particles are such, that being greatly rarified and agitated (by the Heat occafioned by Attrition or rubbing of them) they fly off to a certain fmall Distance,. but not beyond the Sphere of the Body's Attraction; and therefore by this Attraction they are obliged to return again to their old Quarters.

A. Why then, I fuppofe, it is by their flying off, that light Bodies, as Feathers, Hairs, &c. are repelled from the electrical Body, and by the Return of thofe Particles, they are again inftantly compelled back and attached to the faid Body.

B. Yes, that is the Cafe as understood at prefent; but for the real Cause of Attration or Gravity in general, Sir Ifaac Newton profeffeth himself entirely ignorant of it; even though he makes this Principle of

Attra

Of Transparency and Opacity.

79

Attraction or Gravity the Bafis of all his Philofophy: Wherefore let us now difcourfe of fome other Properties of natural Bodies.

CHA P. X.

Of Transparency and Opacity, Denfity, and Rarity, Hardness and Softnefs, Rigidity and Flexibility, in Bodies.

AIR, I thank you for those Instructions concerning Gravity and AttraEtion, and thall be very glad to hear you farther on the other Qualities of Bodies; and first, what may we understand by the Transparency and Opacity of Bodies?

B. Transparency is that Quality of certain Bodies, whereby their Substance is enlightned, and Objects appear vifible through the fame, as in Glafs, Chryftal, &c. and therefore those Bodies are called Transparent, Pellucid, or Diaphanous, all which Terms imply the fame Thing. Opacity is the Quality oppofite hereto; and those Bodies are faid to be Opake, whose Matter is not transparent, or through which Objects are not visible.

A. Pray

A. Pray what is the Cause of these Qualities?

B. The Caufe of Transparency is owing to that Conftitution of Bodies which admits the Rays of Light to pass through their Pores in right Lines, and in all Directions whatever; and on the contrary, Opacity is the Effect of Light obftructed in its Paflage through Bodies, or of that which is not tranfmitted in right Lines.

A. It feems to follow from hence, that the Matter of tranfparent Bodies must be very little in Proportion to their Pores.

B. Very little indeed; fo little, that a celebrated Philofopher queftioned whether the Quantity of Matter in Glafs were more in Proportion to its Porofity, than one Grain of Sand to the Bulk of the whole Earth. A. In what Proportion are Bodies Tranfparent?

B. In Proportion to the Rarity and Denfity of Matter.

A. What do you call the Rarity and Denfity of Matter ?

B. By Rarity is understood the Thinness of the Particles of Matter; and by Denfity, the Thickness thereof; in Refpect of the Bulk of Bodies.

A. Then the Denfity of Bodies hath Regard both to their Matter and Magnitude, I understand by this; but pray what Proportion?

2

B. The

Of Denfity, Rarity, &c.

81

B. The Denfities of two Bodies are in a Proportion compounded of the direct Proportion of their Quantities of Matter, and a reciprocal Proportion of their Magnitudes.

A. I believe I could better apprehend you, if you'd please, Sir, to exemplify this Matter.

B. I will: Thus fuppofe A be a Body which hath 8 Parts of Matter, and Degrees of Magnitude; and B be a Body having 2 Parts of Matter, and 10 Degrees of Magnitude; then the Denfity of A will be to the Denfity of B, as x 48,

that is,

A

B

៖

, or A: B:: 8: 1; therefore

the Body A's Denfity is 8 Times greater than the Denfity of the Body B.

A. Pray, Sir, what Means are those Qualities most of all increased or diminished by ?

B. Heat and Cold; for Heat by dividing, extending, and expanding the Particles of Bodies, doth attenuate and rarify them, and this is called Rarefaction; on the contrary, Cold by uniting, and contracting the Particles of thin Bodies, doth thicken and condense them, and this is called Condenfation.

A. Pray do not the Hardness and Softnels of Bodies arife from their Density and Rarity?

G

B. No,