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ed primary air to escape freely to gain its enlargement. Freely I say, for it seems

perpetually, tho' insensibly escaping; because, a conftant Attrition of the revolving glafs is abfolutely necessary to keep it to its full height : When once that attrition is discontinued, the accumulation on the prime conductor, &c. is almost instantly at an end;, consequently it was before that, constantly escaping. This doctrine concerning its retention when accumulated seems likewise to be confirmed, by observing, that when the barometer is highest, and the air consequently most elastic, the accumulation is then greatest, and continues the longest after the attrition is over. -But although these confiderations alone fufficiently evince, that the elasticity of the circumjacent air is the only reason why the electrical æther in those experiments is so retained, as to accu* .mulate on the included bodies; yet many

others might be produc'd to prove it were it neceffary, and were it possible to conceive any other agent fo ready at hand; and fo capable of confining it down to the electris'd body.'

122. But as Mr. Hauksbee, observes on another occasion, the best proof that can

be given of the truth of any hypothesis, is, ' that the experiments made for that end do • all of them and every way agree : That

trying Nature on one side, and on the other, yet every way (if the hypothesis be right) The ftill confefies the same thing. Thus with

{ respect

ز

ra,

! respect to the nature of sounds; it is demon

strable, that the air is a proper vehicle or me

dium for the propagation of them; because • sounds do not only lessen and grow weakers

according to the degrees of the Air's rare, faction ; but also become more intense and strong, according to the degrees of its condensation.' Mr. Hauksbee's Exp. p. 88. 2d. Edition.

By way of illustration therefore, or ther to put it beyond dispute, I shall proceed to shew that the former arguments are just, concerning the retension of the electrical fluid by means of the repelling spring of the fura rounding air; and that the fame thing is cona firmed by the removal of it.

123. Provide a proper conductor to the inside of a glass vessel, and exhaust the air, then electrise that conductor, and immedia ately á light appears in the vacuum, I 24

"On the bottom of a tall frame of « wood are two cups of glass placed, partly · filld with mercury, in which are immersed • the two ends of a long incurvated glass tube, • in each part whereof, the quicksilver rises • above that in the bason, to the height of a

bout 30 Inches; and all the internal part of • the tube above the quicksilver, is a vacuum, • or a space, as void of air, as can be made,

perhaps, by art.--I lay a wire from the & Barrel (or prime conductor of the machine,) to the mercury in one of the glasses, which

I 2 conducts

over the

the top;

& conducts the electricity to the tube. The

globe is whirl'd round, and behold! How

quick the lightning flies from the mercury . • into the vacuum of the tube ? In that, how <strong, how vivid, how sensibly, and how

quick it moves thro' that long space of the & tube! Afcending, in one part, running

and down the other leg of the • tube, in an apparent rivulet of fire. • When I pụt my finger on the barrel, to in

tercept the fluid, it flows no longer in the « tube. My finger removed, the torrent of « fire rushes on, as before, with an unequal, un• dulating kind of motionti' In that ex, periment, the conductor to the vacuum, was the column of quicksilver. 125. An experiment of this kind

may

be commodioufly made with the exhausted glass receiver, on the air-pump, where the conductor to the vacuum may be the wire, which is made use of to drop the guinea and feather both at once.

And though art may double or triple the quantity and power of electricity, by combining together the action of several globes under attrition, at once, in the fame machine ; yet, to what degree focver it be increased,' 'twill be in vain to attempt to turn fuch streams of light into a vessel that is already filled with air.

+ See Mr. Martin's Gent, and Ladies Philosophy, Vol. ist. p. 321.

6

126. But in many other experiments, if that grand obstacle (the air) be removed, even the glass it self seems as easily peryaded as other gross bodies, and the condensed æther, by means of its fubtilty and great propensity to obtain an equilibrium, instantly escapes through the glass into the vacuum, as freely as if nothing were in the way.

For, 127. · Exhaust a glass globe, and whirl it • round briskly on its axis with the palm of a • hand applied to the surface, and presently

you have a light in the glass, so great, that a

large print may without much difficulty be * read by it.' See. Mr. Hauksbee, p. 45.

128. I took a long glass, (faid he) whose ' air was exhausted, and which had lain by in « that state above six months. After I had • rubb'd this glass a little with my hand, to • clear it of all moisture on the surface, I held

it over the unexhausted globe, which was then • in motion, and at the same time also I

gave • it ( viz. the unexhausted globe ) an attrition ( with my hand; upon which there were im

medịately large and surprising flashes of light · produced in the long glass

, though it neither touched the moving globe, nor was provok'd . it self by any immediate sensible attrition.' Mr. Hauksbee, p. 80.

129. Again, at p. 83. of the same Author. • I took a large receiver, within the body of

which I fix'd another. Their axes were pa-, Fallel to the horizon. The outward surface,

of

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6 of the inner glafs was at least an inch dif

tant from the inner furface of the outward 6 one. Each glass was turned by its own par

ticular wheel, so that either both, or but one might turn at the same time. Before the

glaffes were thus adapted to each other, the « innermost was exhaufted of its air; and then

being plac'd in the machine, I order'd that « wheel only to be turn'd which gave motion « to the great glass; the effect was, that a light

appeared and spread it felf in numerous bran!ches all over. This done; I caus'd the • other wheel to be turn'd, viz. that which

gave motion to the included glass; and then • the light became much more considerable,

and, I think, the greateft that has yet been produced in any experiment made on this subject'. I zo. Farther on.

• I observed farther, • that tho' the effluvia seemed to be equally, 6 diftributed on the outward surface of the 6 inward moving glafs ; yet the light appeared . most vigorously on that fide of it next the "attrition. And when either of the glasses,

was at reft, the other continuing in motion,

(I say eithér ;, for upon trial I found very • little difference either way ; ) the appearance • of the light would remain a confiderable time · within the exhausted glass, till the effluvia,

of the other were no longer capable of acting. s with a force upon it, requisite to produce, the effect'.

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