which appears when a body is rapidly and repeatedly struck or when heated beyond a certain point, as when flint and steel are struck together, etc., to vibrations of the parts of the body so rapid as to throw off the particles which, according to Newton's idea, occasion the sensation of light. With these he also classed electric sparks, saying that the 'electric vapor' excited by rubbing glass dashes against a strip of paper or the end of the finger held to it, is thereby so agitated as to cause it to emit light. He thought the light from glowworms and putrefying matter was of the same kind as the above, and said that the light seen at night in the eyes of certain animals, cats for instance, is 'due to vital motions.'
Regarding true luminous flames Newton's ideas were nearer those of the present time. He wrote "Is not fire a body heated so hot as to emit light copiously? For what else is a red hot iron than fire? And what else is a burning coal than red hot wood?" "Is not flame a vapor, fume or exhalation heated red hot, that is, so hot as to shine? For bodies do not flame without emitting a copious fume, and this fume burns in the flame. Metals in fusion do not flame for want of a copious fume." "All fuming bodies, as oil, tallow, wax, wood, etc., by fuming waste and vanish into burning smoke." 'Put out the flame and the smoke is visible, it often smells; and the nature of the smoke determines the color of the flame.' "Smoke passing through flame can not but grow red hot, and red hot smoke can have no other appearance than that of flame."
During the hundred years, more or less, following the publication of Newton's views there was little change in the prevailing theories. Stahl said 'flame is light' liberated from bodies in the act of combustion, and that light and heat are the constant attendants of flre; fire combined with combustible matter was 'phlogiston.' Scheele said light, heat and fire are combinations of air and 'phlogiston.' Lavoisier thought flame to be light disengaged from air, with which it had been in combination, and this idea seems to have been adopted by most of the French chemists.
There might be mentioned in this connection the queer ideas regarding our being able to see objects, and the emission of light by incombustible bodies, which were held during the latter half of the eighteenth century. As expressed by Macquer, and quoted by Fourcroy,[1] "The vibrations (under the impulse of more or less heat) dispose the particles (of bodies) in such a manner that their faces, acting like so many little mirrors, reflect upon our eyes the rays of light which are in the air by night as well as by day; for we are involved in darkness during the night for no other reason but because they are not then so directed as to face our organs of sight."
- ↑ Fourcroy's 'Chemistry,' press date 1796.
