4 THE PROPERTIES

with which it quits them, and diverges from its focus : forwhen the quantity of fire in any body is greater than in theatmosphere or other surrounding bodies, it quits that body,and removes from it, spreading itself equally on all sides;its activity diminishes in proportion to its distance from thebody which it quitted, and its extension depends on itsexcess over the external fire, on the time of application,the superficies, quality, and mass, of the surrounding bo-dies.

io. Having thus examined the various circumstances thattend to modify the effects of fire, let us now consider thesemodifications with respect to the ignition and destructionof combustible bodies. Whether the ignited motion excitedin a combustible body appears under the form of flame, orburning coal, it is always produced in two manners; eitherly applying to the combustible body a quantity of external fire,or by increasing the motion of the fire that exists in it.

ix. From the facility with which combustible bodies takefire, they may be ranged in two classes. In the first maybe placed spirits of wine rectified, sulphur, &c. in which, byapplying fire to any one part, it is communicated to thewhole mass, so as to produce a total destruction. In thesecond class may be ranked coal, the greater part of vege-tables, &c. in which fire can be propagated only by a con-tinued application; by increasing the motion in the ignitedparts; or by placing the bodies in such a manner, that thefire, in escaping from the burning particles, may meet almostinstantaneously those not yet ignited.

12. The degree of ignition varies according to the dif-ferent qualities of the bodies themselves ; for a greater degreeof fire is requisite in proportion to the rarefaction of thesurrounding air, or to the difficulty of removing from theignited body smoke or other substances which do not serveas. food to fire. In both these cases, it is necessary eithercontinually to apply fresh fire, or to increase the motionof that which already exists in the body.

13. From these premises, it may be inferred, that fireadmits of infinite diversity in its effects, arising not onlyfrom the time of application, the superficies, quality, andmass, of the bodies to which it is applied, and which sur-round it, but even from the modifications that ensue fromthe quantity and velocity of the ignited matter.

14. Where-