Now all these, although they bear a share in the discharge, yet have they distinct intentions, and different offices in the composition. From Brimstone proceedeth the piercing and powerful firing; for Small-coal and Petre together will onely spit, nor vigorously continue the ignition. From Small-coal ensueth the black colour and quick accension; for neither Brimstone nor Petre, although in Powder, will take fire like Small-coal, nor will they easily kindle upon the sparks of a Flint; as neither will _Camphire_, a body very inflamable: but Small-coal is equivalent to Tinder, and serveth to light the Sulphur. It may also serve to diffuse the ignition through every part of the mixture; and being of more gross and fixed parts, may seem to moderate the activity of Salt-petre, and prevent too hasty rarefaction. From Salt-petre proceedeth the force and the report; for Sulphur and Small-coal mixed will not take fire with noise, or exilition, and Powder which is made of impure and greasie Petre hath but a weak emission, and giveth a faint report. And therefore in the three sorts of Powder the strongest containeth most Salt-petre, and the proportion thereof is about ten parts of Petre unto one of Coal and Sulphur. But the immediate cause of the Report is the vehement commotion of the air upon the sudden and violent eruption of the Powder; for that being suddenly fired, and almost altogether, upon this high rarefaction, requireth by many degrees a greater space then before its body occupied; but finding resistance, it actively forceth his way, and by concusion of the air occasioneth the Report. Now with what violence it forceth upon the air, may easily be conceived, if we admit what _Cardan_ affirmeth, that the Powder fired doth occupy an hundred times a greater space then its own bulk; or rather what _Snellius_ more exactly accounteth; that it exceedeth its former space no less then 12000 and 500 times. [SN: _The cause of Thunder._] And this is the reason not only of this fulminating report of Guns, but may resolve the cause of those terrible cracks, and affrighting noises of Heaven; that is, the nitrous and sulphureous exhalations, set on fire in the Clouds; whereupon requiring a larger place, they force out their way, not only with the breaking of the cloud, but the laceration of the air about it. [SN: _The greatest distance of the Clouds._] When if the matter be spirituous, and the cloud compact, the noise is great and terrible: If the cloud be thin, and the Materials weak, the eruption is languid, ending in coruscations and flashes without noise, although but at the distance of two miles; which is esteemed the remotest distance of clouds. And therefore such lightnings do seldom any harm. And therefore also it is prodigious to have thunder in a clear sky, as is observably recorded in some Histories. [Sidenote: _The cause of Earthquakes._]