3 It is impossible for one and the selfe same body to possesse at one time two places; It followeth therefore, that a dense body rarified, and made thin, eyther by actuall or potentiall fire, requireth a greater quantity of room to be conteyned in, then it did before. Hence it is, that if you lay your hand upon a glasse, hauing a straight mouth reuerst into a dish of water, it rarifieth the ayre contayned therein, and makes it breake out thorough the water in bubbles. Also, that gunpowder inclosed in the barrell of a gun, being rarified by fire, applied unto the touch-hole, it seeketh a greater quantity of roome, and therefore forceth the bullet out of the barrell. This is called violent motion. 4 According unto the strength and quantity of a dense body rarified, and according unto the forme and length of its inclosure, it forceth its compresser further or neerer at hand. Thus much shall suffice to haue spoken concerning the _Præcognita_: Now I will passe _ad majora, & ad magis necessaria_: to wit, those necessary Instruments, and seuerall sorts of Ingredients, that ought to be had in readines. As for the instruments they are these; Morters and Pestles, Serces, also seuerall sorts of Formers, Paper, Parchment, Canuas, Whipcord, strong binding thread, Glew, Rosin, Pitch, with diuers vessells meet to contayne and mingle your compositions in. The ingredients likewise are chiefly these, Saltpeter, Rochpeter, Sulpher, Charcoale, good Gunpowder, Filings of steele, oyle of Peter, and Spirit of wine. _Instructions for chusing your ingredients._ Saltpeter is very good, if that being layd upon a board, and fire put to, it rise with a flamed ventosous exhalation, raysing no scum, nor leauing no pearle, but onely a blacke specke burnt into the boord. The best brimstone, is quick brimstone, or liue sulphur, and that sort is best that breaketh whitest; if this cannot be gotten, take of the whitest yellow brimstone. The best Coales for use are the sallow, willow, hazel and beech; onely see they be well burnt. Euery of these ingredients must be powdred finely and searsed. All kindes of gunpowder are made of these ingredients imposed, or incorporated with vineger, or aquauitæ, and afterward grayned by art. The Saltpeter is the Soul, the Sulphur the Life, and the Coales the Body of it. The best sort of powder may be distinguished from others, by these signes: 1 If it be bright and incline to a blewish colour. 2 If in the handling it proue not moyst but auoydeth quickely. 3 If being fired, it flash quickly, and leaue no dregs nor setlings behinde it. _A device to try the strength of divers sorts of Gunpowder._ [Illustration]