_Boerhaave_ has another observation, which seems to prove to a demonstration, that fire or heat is caused by an _attrition_ of the air; which observation I shall give in his own words. “A cannon ball, shot in the winter time, will fly 600 feet in a minute through the cold air, which makes a greater resistance than any wind, the most rapid of which only moves 22-1/2 feet; hence it appears how much _friction_ the ball must have undergone in its passage, which by the way did not proceed in a right line, but by its whirling motion continually describes a cycloid with every point of its body. When it falls, it is found quite hot; notwithstanding in its whole passage, it had continually met with cold air. This heat could not have arisen from the flaming gunpowder, whereby it was exploded, since it only remained in that flame, an incredible small space of time, in which it is by no means credible so solid a body should have acquired such a heat; which is much more naturally accounted for from the great attrition of the ball, driven with such a velocity through the air, and repelled by a wind, which is above 27 times swifter than the strongest wind hitherto observed.” _Boerhaave_’s Chym. by _Shaw_, Vol. 1. pag. 244. I shall mention one more experiment from Mr. _Boyle_, which proves that _air_ is not only convertible into _light_; but that it may afterwards be forced through glass, and thereby the same _vacuum_ be made that is by an air pump. “Liquid phosphorus being put into a vial, when it was disposed to shine in the dark, the cavity of the vial above the liquor seemed to be full of whitish fumes, though at other times transparent. The vial, when close stopped, was not luminous in the dark, but the light or flame appeared as soon as it was exposed to the air, and the vial was unstopped; and that the occasion and propagation of this flame depended on the contact of the air, appeared, since agitation would not kindle it, but when the bottle was unstopped, the kindled flame would gradually be propagated downwards; the flame always appeared most vivid the nearer the air, and when it was extinguished, it first disappeared in the bottom, and then expired at the top. When the Vial was unstopped for some time, when it was stopped again, the air that had leisurely insinuated itself would cherish the flame for an hour or two. It was observable, that when the air had been long pent up with this shining liquor, its resistance would be so weakened, that when the vial was unstopped, the external air would presently rush in with violence, from whence appears the interest of the air in propagating the shining of this liquor. The agitation before the vial was unstopped would not kindle the light; yet when it was opened, it would be increased by it, and even when it was in its dull state, if I poured a little of it upon my hand, and rubbed it with my finger, it would presently become vivid, and emit store of luminous rays, as