WHY A FLAME EMITS LIGHT 91 



. Among the first recorded experiments upon the nature 

 and action of luminous flames are those which were carried 

 out by Sir Robert Boyle between 1660 and 1670. He at- 

 tempted to prove by experiment whether the light from, a 

 flame is like that from the sun, and whether it is corporeal 

 or merely a quality. He allowed a flame to play on metals 

 directly and also when in open and sealed vessels, and be- 

 cause the substance formed a calx and gained in weight, he 

 thought that the light or flame (he uses the term indiscrim- 

 inately) had combined with the metal, and hence it must be a 

 fluid. Boyle also conducted a large number of experiments 

 upon live or "quick" coals, phosphorescent bodies, animals 

 and insects to see the effect of exhausting a receiver in which 

 they were placed, and he seems to have concluded that the 

 lights from live coals, rotten wood and putrefying fish differ 

 not in kind but only in degree. He considered that the 

 increase of light from coals, etc., and the reviving of certain 

 insects when air was readmitted to the receiver indicated a 

 relation between a visible flame and the so-called " vital 

 flame." But he would not commit himself upon the ques- 

 tion of the supposed kinship between the ' * flame ' ' from live 

 coals and rotten wood and the "vital flame" thought to be 

 burning in the hearts of all living beings. 



The interesting views of Sir Isaac Newton are set forth 

 in a number of queries published in his work entitled 

 Optics. As is well known, Newton believed in the ma- 

 terial nature of light, and he asserted that the change of 

 light into matter and of matter into light is an acknowl- 

 edged possibility and of common occurrence. He attributed 

 the light which appears when a body is rapidly and repeat- 

 edly 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, 



