A Crucial In

deavours to prove,) be given by the Prefence of the Earth: and, therefore, the Particulars which he has, with much Sagacity and Industry, discover'd, amount to this; that an Iron Nail, which has long continued in the Direction of North and South, may, by that mere Continuance, receive a Verticity; without the Touch of the Magnet: as if the Earth itself, tho' it operates weakly, by reason of its Distance, (for the Surface, or external Cruft of the Earth, has no magnetic Virtue, according to him,) should yet, in fo great a Length of Time, fupply the Defect of the Loadstone; excite the Iron; and afterwards make it comply, when thus excited. And, again; that if ignited Iron be quenched, pointing in the Direction of North and South, it alfo receives a Verticity, without the magnetic Touch; as if the Parts of the Iron, put in Motion by the Ignition, and afterwards contracting themfelves, in the very Inftant of quenching, were more fufceptive and fenfible of the Virtue rifing from the Earth, than at another Time; and thence become animated but thefe Particulars, tho' well obferved, do not clearly prove his Point.

125. This may be a Crucial Inftance in the prefent Cafe. Mark the ftance in the Poles of a Terella, and place them Eaft and Weft; then lay an unCaje. touched Needle thereon, and let it remain for fix or feven Days. The Needle, no doubt, whilst it lies upon the Magnet, will quit the Poles of the World; and conform to thofe of the Magnet; and, therefore, as long as it remains thus, it points East and Weft. But if the Needle shall be found, when removed from the Terella, or Magnet, and placed upon its Pin, immediately to turn North and South; or by degrees to move into that Direction; then the Earth's Prefence is to be admitted for the Caufe: but if it turns as before, East and Weft; or lofes its Verticity; then that Caufe fhould be accounted doubtful; and farther Enquiry be made a.

The Substance of the Moon; the Question.

The Crucial Inftances therein.

126. In like manner, let the Subject of Enquiry be the Subftance of the Moon, to determine whether it be rare, flamy, or aerial, as many of the ancient Philofophers conceived; or folid and denfe, as Gilbert, with many of the Moderns, and fome of the Ancients, contend. The Reafons of the latter Opinion depend chiefly upon this, that the Moon. reflects the Rays of the Sun; and that Light appears to be reflected by none but folid Bodies. And, therefore, if there are any, thofe may be reckon❜d Crucial Inftances, with regard to this Subject, which demonstrate that Reflection may be made by a rare Body, as Flame is, provided it be fufficiently thick.

127. And, doubtlefs, one Caufe of the Twilight, among others, is the Reflection of the Sun's Rays from the upper Part of the Air. We fometimes fee the Rays of the Sun reflected in clear Evenings, from the Edges of dewy Clouds, with a more refplendent Brightness than

See the Sylva Sylvarum, under the Article Magnetism,

that

that afforded by the Body of the Moon; and yet it is not certain that thofe Clouds are collected into a denfe Body of Water. So likewife, we fee that the dark Air behind a Window, by Night, reflects the Light of a Candle, as well as a dense Body.

128. Experiment alfo fhould be made of tranfmitting the Sun's Rays thro' a Hole, upon a dufky, or blue Flame: for the free, open, and unconfined Rays of the Sun, falling upon obfcure Flames, feem to deaden them; and make them appear rather like white Fumes than Flames ". And these are Inftances that occur for the prefent, to fhew the Nature and Ufe of the Crucial Inftances, with regard to this Subject: tho', doubtless, better may be found for the purpose. But let it always be observed, that a Reflection from Flame is not to be expected, unless the Flame be of fome Depth, Thickness, or Body; for otherwife it inclines to Transparency. But this is to be held certain, that Light, in an uniform Substance, is always either received and tranfmitted, or reflected c.

129. Again; let the Nature fought be Projectile Motion, as the Motion The Motion of of a Dart, an Arrow, a Bullet, &c. thro' the Air. This Motion the Projectiles. Schools, according to their Cuftom, have fightly pass'd over, esteeming it fufficient to diftinguish it by the Name of Violent Motion, from that they call Natural; and for the firft Impulfe, fatisfying themselves with this, that two Bodies cannot be in the fame Place, otherwise a Penetration of Dimensions would enfue; without at all regarding the continued Progrefs of this Motion ".

130. The Crofs-Way here lies thus. This Motion is either caused by The Crossthe Air propelling and gathering itfelf behind the Projectile; as Way thereof. the Water does behind a Ship; and the Winds behind the Chaff which they blow away, &c. or elfe by the Parts of the Body not fuftaining the Impulfe; but urging forwards to relax themselves, by Succeffion, from the impelling Force. Fracaftorius, and nearly all those who have made any fubtile Enquiry into this Motion, take the first Path: nor can it be question'd, but the Air has fome Share herein; tho', doubtlefs, the other Motion is the true one; as appears from numerous Experiments.

131. But, among the reft, this may be a Crucial Inftance to the pur- A Crucial Inpofe; that a Piece of ftubborn Iron Plate, Wire, a Quill, or the like, fance in the being bent in the middle by the Fingers, will when let go, fpontaneously Cafe. fly back. Now, it is plain, that this Motion cannot be attributed to the Air, collecting itself behind the Body; because the Origin of the Motion is in the Middle of the Plate, Wire, or Quill; and not in the Extremities, or Ends .

b See Sir Ifaac Newton's Optics, paffim; and Dr. Hook's Lectures of Light. See Mr. Boyle of Colours, and Sir Isaac Newton's Optics.

See Vol. III. p. 607.

See the Eay upon Violent and Projectile Motion, Vol. III. p. 606, &c.

into Flame.

The Expansion 132. Again; let the Nature fought be the rapid and powerful Exof Gunpowder panfion of Gunpowder into Flame; whereby fuch vaft Maffes of Building are overturn'd; and fuch great Weights thrown to a confiderable Diftance; as we fee fee in the Springing of Mines, the Firing of Mortars, &c. The double Road here lies thus. This Motion is either excited by the mere Appetite of the Body, to dilate itself, after it is fet on Fire; or by a mix'd Appetite of the crude Spirit of the Nitre, which, with great Rapidity, avoids and flies from Fire; and violently bursts out from the midft thereof, as from a Prifon. The Schools, and the vulgar Opinion, here confider only the former Appetite for Men have thought they philofophized notably, in afferting Fiame to be endowed, by the Form of the Element, with a neceffary Quality of poffeffing a larger Space, than the fame Body poffefs'd when it had the Form of Gunpowder; and that this Motion must from thence enfue.

The Cross-
Road.

The Crucial Inftances in the Cafe, of two Kinds.

133. But here they obferve not, that, tho' this be true, upon Suppofition the Flame is ready generated; yet the Generation of Flame may be hinder'd by a Mass of Matter, able to fupprefs and fuffocate it; fo that the Thing may not be reduced to the Neceffity they fpeak of. Indeed, as to the Neceffity of the Expanfion, and the confequent Explofion, or Difcharge of the Bullet, or obstructing Body, when the Flame is generated; they judge rightly: but this Neceffity is plainly avoided, if the folid Body fupprefs or prevent the Flame, before it is generated. And we fee, that Flame, efpecially in its first Generation, is foft, and gentle; requiring a Cavity wherein to play, and exert it felf; whence fuch a Violence cannot be attributed to Flame of itfelf.

134. Without Doubt, the Generation of this flatulent Flame, or, as it were, fiery Wind, arifes from a Conflict of two Bodies, that have very different Natures; the one being highly inflamable, which is the Nature powerful in Sulphur; the other highly impatient of Flame, which is the Cafe in the crude Spirit of the Nitre: whence a wonderful Conflict arifes; the Sulphur immediately catching all the Flame it is capable of; and the Spirit of Nitre burfting forth with all its Violence; at the fame Time dilating itself, as Air, Water, and all crude Bodies do, when affected by Heat; and whilft it breaks forth, and flies off every Way, it blows up the Flame of the Sulphur, as it were, with internal Bellows: the Willow-Coal, in the Compofition, ferving for little more, than to incorporate, and commodiously unite, the Sulphur and Salt-Petre together f.

135. But the Crucial Inftances upon this Subject might be of two Kinds; the one with regard to thofe Bodies which are most inflammable, as Sulphur, Camphire, Naphtha, &c. with their Mixtures; and which, if not otherwife

See the Essay upon the Caufe of the Motion of Explofion in Guns and Gunpowder, Vol. IIL

p. 611.

otherwife hindered, take Flame more easily than Gunpowder: from whence The first.
it is plain, that the Appetite of Inflammability does not, of itfelf, pro-
duce this ftupendous Effect.

136. The other Kind is, of thofe Bodies which refift and repel The fecond.
Flame, as all Salts do for we find, when thefe are thrown into the
Fire, a watery Spirit breaks out, with a crackling Noife, before they
take Flame; which the more stubborn kind of Leaves & do alfo, in a
gentler manner; their aqueous Part burfting forth before their oily Part
takes Flame. But this appears more eminently in Quickfilver, which is
not improperly called a foffil, or metallic Water: for Quickfilver, without
taking Flame, almoft rivals the Force of Gunpowder, by bare Eruption
and fimple Expanfion; and being mix'd with Gunpowder, is faid to increase
the Strength thereof i.

Flame.

137. Laftly, let the Subject of Enquiry be the Tranfitory Nature of The tranfitorg Flame, and its momentary Extinction: for the flamy Nature does not with Nature of us appear permanent, and at a ftay; but to be momentarily generated, and presently after extinguished again. It is manifeft, that in the Flame here fuppofed to be continued and durable, the Duration is not of the fame individual Flame; but happens by a Succeffion of new Flame, regularly generated, without continuing numerically the fame; as easily appears from hence, that if the Fewel or Aliment be taken away, the Flame prefently goes out.

Road.

138. The two Ways in this Subject lie thus. The momentaneous The CrossNature proceeds either from a Remiffion of the Cause that first produced it, as in Light, Sounds, and thofe called violent Motions; or elfe from hence, that Flame, in its own Nature, cannot fubfift here below without fuffering, and being deftroy'd, by the contrary Natures around it. 139. This, therefore, may be a Crucial Inftance in the Cafe. We 4 Crucial Infee in great Conflagrations, that Flames will afcend to a confiderable fance in the Cafe. Height; for the wider the Bafis of the Flame, the higher its Vertex rifes; and therefore Extinction appears to begin about the Sides, where the Flame is comprefs'd, and oppofed by the Air: but the inner Parts of the Flame, untouch'd by the Air, and every where furrounded by other Flame, remain numerically the fame; without being extinguifh'd, till they come to be gradually fqueezed by the Air diffufed about the Sides and therefore all Flame is pyramidal, or large in its Bafis, about the Fewel, but fharp at the Vertex; the Air being its Antagonist, and not fupplying Fewel. But the Smoak, which is narrower about the Bafis, dilates itfelf in afcending, and becomes like an inverted Pyramid; because

Such, in particular, as Ivy, Bays, &c.

That is, fuppofing the Quick fiver clofe confined and heated, as it might be, to fhew the
Thing, in a Gun-barrel, with the Touch-hole stopped, and the Charge hard rammed down with
Paper, &c.

Confider of the A. rum Fulminans; the common Pulvis Fulminans; the Ways of making
the Mercaring; and the Means of increafing the Strength of Gunpowder, by Salt of
Tartar, Prepitated Metals, &c.

because the Air receives Smoak, but compreffes or fqueezes Flame. And let no one fondly imagine, that Flame is Air fet on fire; for Flame and Air appear perfectly heterogeneous *.

A fecond Cru140. We might have a more accurate Crucial Inftance to this Purpose, cial Inftance. if the Thing could be manifefted by Flames of different Colours. Take, therefore, a fmall metalline Difh, and fix therein a fmall Wax-taper lighted; fet the Dish in a wider Veffel, and pour Spirit of Wine round it, in a moderate Quantity, fo as not to touch the upper Rim or Edge of the Dish; fire the Spirit of Wine, and this will exhibit a bluith Flame; but the Wax-taper one that is yeliower: and now let it be obferved, whether the Flame of the Taper remains pyramidal; which may eafily be diftinguished through the blue-colour'd Flame of the Spirit of Wine, (for Flames do not prefently mix as aqueous Liquors do ;) or whether it tends not to a spherical Figure; as there is nothing present to deftroy or comprefs it. And if the latter prove to be the Cafe, it may be held certain, that Flame remains numerically the fame, fo long as it is furrounded by other Flame, without feeling the hoftile Effect of the Air

Thele Crucial

141. And fo much for Crucial Inftances; upon which we have been the Inftances, why fuller, that Men may gradually learn and accuftom themselves to judge fo largely of Nature by Instances of the Crofs; and Experiments of Light; and not by probable ReasoningsTM.

dwelt upon.

Inftances of
Divorce.

Their Use.

APHORISM XXXVII.

142. In the fifteenth Place, among Prerogative Inftances, come the Inftances of Divorce; which indicate the Separation of fuch Natures as frequently meet or come together. Thefe differ from the Subjunctive Kind, or Inftances fubjoined to Accompanying Inftances; because thofe indicate the Separation of a Nature from a Concrete, wherein it familiarly appears ; but these the Separations of one Nature from another. Thefe alfo differ from Crucial Inftances, as determining nothing; but only admonishing us of the Separability of one Nature from another.

143. Their Ufe is to difcover false Forms; and to diffipate fuperficial Notions and Speculations arifing from obvious Things; fo that they add, as it were, Ballaft to the Understanding.

k Dr. Hook's Lectures of Light, paffim.

This is a fubtile Experiment, and of great Moment. See Vol. III. p.

144. For

Which are Endlefs, and lead to no folid Determinations. And by this Time, if the Reader has been tolerably attentive and diligent, he will have a clear Perception of the fuperior Excellence and Use of this Doctrine of Prerogative Instances; and be enabled, in fome tolerable Degree, to profecute Enquiries by their Means; in order to the full Investigation and Difcovery of the Forms of Things. But the Author's larger Examples, that occur in the third Volume, will render the whole ftill plainer.

See above, Aph. 33 and 34.

• See below, §. 144.