OF GUN-POWDER. 21

alkali that easily attracts moisture, and falls in deliqulum whenexposed to an air abounding in vapour.

56. Since the fluid generated at the burning of powderpreserves its elasticity for a long time, it follows, that at theinstant of explosion its elastic force must be the greatest.In proof of this, let a barometer be fixed to the pneumaticmachine; and, having exhausted the receiver, apply fire tothe thimble. (Fig. I.) At the instant of explosion, themercurial gage descends rapidly, then rises; and, after someundulations, seems to fix, fora time, below the point whereit was before the explosion. This apparent fixation shewsthat the elastic fluid is reduced to the temperature of the air jand the sinking of the mercury, in the beginning of the ex-periment, proves that the force of this fluid is much greaterat the instant of the explosion than afterwards. If, instead ofburning the powder in vacuo, it be fired in the open air, theelasticity of the fluid will be greater during the application offire than when its effects totally cease.

57. From these premises (32, 34.) it may be inferred,that the quantity of the permanent fluid is always in proportionto the quantity of nitre contained in the powder, since italone produces the fluid ; as sulphur and charcoal, whenburned, produce none. Other experiments might be madeto confirm this, by burning different quantities of powder, ofthe fame or different qualities, under the receiver of thepneumatic machine; exhausting the air to the fame degreein each experiment, and observing the apparent fixed point ofthe mercurial gage: it would then be seen that it sensiblyfalls or rises in proportion to the saltpetre contained in thedifferent quantities of powder.

58. Though the penetration of the bullet into the plank,and the apparent fixed point of the mercurial gage, (55,56, 57.)be the effects of the permanent fluid generated from thesaltpetre, yet they ought not to be attributed solely to that;as the smoke and the common air contained in and betweenthe grains of powder, are also rarefied by the action of thefire.

59. From all experiments hitherto made upon smoke, itis found to be elastic while hot; wherefore it is reasonable toconclude, that the smoke produced at the explosion is one ofthe causes that concur to give it force: but when cold, it notonly ceases to be elastic, but even absorbs a portion of thepermanent fluid. It is impossible, at the explosion, by fepa-

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