l 5 THE PROPERTIES

mixture of mealed powder, sulphur, and saltpetre. Let twoor three fuses, of the same size, be filled with compositionof different degrees of strength; after being set on sire, letthem be buried in earth equally stiff, or covered in any othermanner, so as to prevent the escape of the smoke; it will befound, on taking them out after some minutes, that thequantity of composition which is consumed is in proportionto its strength ; since the density of the smoke was the same,as the ground was equally solid. Hence, it is evident, thatthe greater the force of the fire, the greater is the consump-tion of the composition. Care sliould be taken not to makeit so strong as to burst the fuses.

48. Having ascertained those two properties, it remains todemonstrate, that when fire is applied to grains of powder,the inflammation of the contiguous grains, and the destruc-tion of each individual grain, takes place progressively;(35. N° 3.) and that the velocity with which fire spreadsitself on all sides to inflame the contiguous grains, is greaterthan that with which it penetrates into the substance of eachgrain. It is too obvious to need insisting on, that all mo-tion, however rapid or short it may be, takes up a certaintime; though to us, from the shortness of its duration, itappears instantaneous : consequently, the inflammation andentire destruction of powder produced by the action of fire,communicating itself to every thing around, like rays from acentre, must necessarily take place in a determined space oftime; which varies according to the strength of the fire, theproportion of the ingredients, the nicety of the mixture,and the size of the grains.

When a sufficient degree of fire is applied to one grain ofpowder, it first acts upon the surface, and then penetratestowards the centre. (43.) As the surface burns, a flame isexcited which catches the nearest grains; if the degree ofheat be sufficient, and the surrounding air not too muchrarefied. In the mean time, the fire which attacked the firstgrain, continues its action towards the centre till it be totallyconsumed. There are then two distinct actions in the in-flammation and the total consumption of powder : the first isthe expansion of the inflamed fluid; which, spreading itselffrom the surface of the burning grains, surrounds the conti-guous ones: the second is the penetration of the fire from thesurface of each grain towards its centre. But the flame al-ways spreads with more rapidity between the intervals of the

other