^yHY A FLAME EMITS LIGHT. 211 



which appears when a body is rapidly and repeatedly struck or when 

 heated beyond a certain point, as when flint and steel are struck to- 

 gether, 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 sensa- 

 tion 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.' 



Eegarding 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 jSTewton's views there was little change in the prevailing theories. 

 Stahl said 'flame is light' liberated from bodies in the act of combus- 

 tion, 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 regard- 

 ing our being able to see objects, and the emission of light by incom- 

 bustible bodies, which were held during the latter half of the eighteenth 

 century. As expressed by Macquer, and quoted by Fourcroy,* "The 

 vibrations (under the impulse of more or less heat) dispose the par- 

 ticles (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 dur- 

 ing 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. 



