sulphur, camphire, naphtha, &c. with their mixtures; and which, if not otherwise hindered, take flame more easily than gunpowder; from whence it is plain, that the appetite of inflammability does not of itself produce this stupendous effect.

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The other kind is, of those bodies which resist and repel flame, as all salts do; for we find, when these are thrown into the fire, a watery spirit breaks out, with a crackling noise, before they take flame, which the more stubborn kind of leaves* do also in a gentler manner; their aqueous part bursting forth before their oily part takes flame. But this appears more eminently in quicksilver, which is not improperly called a fossil, or metallic water; for quicksilver, without taking flame, almost rivals the force of gunpowder, by bare eruption and simple expansion †, and being mixed with gunpowder, is said to increase the strength thereof.

*Such, in particular, as ivy, bays, &c.

That is, supposing the quicksilver close confined and heated, as it might be, to shew the thing, in a gun-barrel, with the touch-hole stopped, and the charge hard rammed down with paper, &c.

Consider of the Aurum Fulminans; the common Pul vis Fulminans; the ways of making the Mercurius Fulminans; and the means of increasing the strength of gunpowder, by salt of tartar, precipitated metals, &c.

Lastly, let the subject of enquiry be the transitory nature of flame, and its momentary extinction; for the flamy nature does not with us appear permanent and at a stay, but to be momentarily generated, and presently after extinguished again. It is manifest, that in the flame here supposed to be continued and durable, the duration is not of the same individual flame, but happens by a succession of new flame regu-! larly generated, without continuing numerically the same, as easily appears from hence, that if the fuel or aliment be taken away, the flame! presently goes out.

The two ways in this subject lie thus. The momentaneous nature proceeds either from a remission of the cause that first produced it, as in light, sounds, and those called violent mo tions; or else from hence, that flame, in its own nature, cannot subsist here below without suffering and being destroyed by the contrary natures around it.

This therefore may be a crucial instance in the case. We see in great conflagrations, that flames will ascend to a considerable height; for the wider the basis of the flame, the higher its vertex rises, and therefore extinction appears to begin about the sides, where the flame is com pressed and opposed by the air; but the inner parts of the flame untouched by the air, and

its

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every where surrounded by other flame, remain numerically the same, without being extinguish→ ed, till they come to be gradually squeezed by the air diffused about the sides, and therefore all flame is pyramidal, or large in its basis, about the fuel, but sharp at the vertex, the air being antagonist, and not supplying fuel. But the smoak, which is narrower about the basis, dilátes itself in ascending, and becomes like an inverted pyramid, because the air receives smoak, but compresses or squeezes flame. And let no one fondly imagine, that flame is air set on fire, for flame and air appear perfectly hete rogeneous*.

* We might have a more accurate crucial instance to this purpose, if the things could be manifested by flames of different colours. Take, therefore, a small metalline dish, and fix therein a small wax-taper lighted; set the dish in a wider vessel, and pour spirit of wine round it in a moderate quantity, so as not to touch the upper rim or edge of the dish; fire the spirit of wine, and this will exhibit a bluish flame; but the wax-taper, one that is yellower: and now let it be observed, whether the flame of the taper remains pyramidal, which may easily be distinguished through the blue-coloured flame

Dr. Hook's Lectures of Light, passim.104-20180

of the spirit of wine (for flames do not presently mix as aqueous liquors do,) or whether it tends not to a spherical figure, as there is nothing present to destroy or compress it. And if the latter prove to be the case, it may be held cera tain, that flame remains numerically the same, so long as it is surrounded by other flame, with-3 out feeling the hostile effect of the air *. MATE

And so much for crucial instances, upon which we have been the fuller, that men may gradually learn and accustom themselves to judge of nature by instances of the cross, and experiments of light; and not by probable reasonings +.

37. In the fifteenth place, among prerogative instances, come the instances of divorce, which indicate the separation of such natures as fre quently meet or come together. These differ from the subjunctive kind, or instances subjoin ed to accompanying instances; because those indicate the separation of a nature from a con

This is a subtile experiment, and of great moment. Which are endless, and lead to no solid determinations. And by this time, if the reader has been tolerably attentive and diligent, he will have a clear perception of the superior excellence and use of this Doctrine of Peroga tive Instances, and be enabled, in some tolerable degree, z to prosecute Enquiries by their means, in order to the full investigation and discovery of the forms of things.

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crete wherein it familiarly appears, but these the separations of one nature from another. These also differ from Crucial Instances, as determining nothing, but only admonishing us of the separability of one nature from another.

Their use is to discover false forms, and to dissipate superficial notions and speculations arising from obvious things, so that they add, as it were, ballast to the understanding.

For example, let the subject of enquiry bet those four natures which Telesius calls chamber fellows, as if they came out of the same room, viz. heat, light, rarity, and mobility, or aptness to motion. Now though these natures seem to be nearly related, yet there are many instances of divorce found among them; for, 1. the air is rare, and ready to motion, but not hot or shining; 2. the moon is lucid, without heat; 3. water is hot, without light; 4. the motion of the magnetic needle in the compass, is swift and nimble, yet that needle is a cold, dense, and opake body; and there are many other examples of this kind*.

Again, let the subject of enquiry be the cor poreal nature, and natural action; for natural action seems to be no where found, but as it subsists in some body or other, and yet with regard

* By comparing this with Aph. 4. of the present part, the use of these Instances, in discovering false Forms, will sufficiently appear.