2IO POPULAR SCIENCE MONTHLY. 



But before explaining this theory more fully and seeing upon what 

 experimental evidence it is based, it would be well to consider its gene- 

 sis and briefly recall the ancient notions regarding 'artificial' light. 



Light was first confused with seeing, and it is said that up to the 

 time of Aristotle men commonly thought they saw by reason of some- 

 thing shooting out from the eyes and coming in contact with objects; 

 the converse of the Cartesian conception of many centuries later, that 

 certain movements in bodies cause them to shoot out minute particles 

 in all directions, which, striking the eye or causing 'globules' of air to 

 strike it, excite vision. 



The fluid nature of fire and the corporeal nature of light, which were 

 believed in throughout the early and middle ages, seem to have been 

 first doubted by Sir Francis Bacon about the end of the sixteenth cen- 

 tury, although he was by no means sure that these conceptions were 

 wrong. Bacon classed together the light from flames, decayed wood, 

 glowworms, silks, polished surfaces, etc., and said that inasmuch as 

 some animals can see in the dark, air has some light of itself. Boer- 

 haave, somewhat later, also expressed doubts as to the substantive na- 

 ture of fire. 



Among the first recorded experiments upon the nature and action 

 of luminous fiames are those which were carried out by Sir Eobert Boyle 

 between 1660 and 1670. He attempted to prove by experiment whether 

 the light from a flame is like that from the sun, and whether it is cor- 

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

 directly and also when in open and sealed vessels, and because the sub- 

 stance formed a calx and gained in weight, he thought that the light 

 or flame (he uses the terms indiscriminately) had combined with the 

 metal, and hence it must be a fluid. Boyle also conducted a large nimi- 

 ber 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 re- 

 ceiver indicated a relation between a visible flame and the so-called 

 'vital flame.' But he would not commit himself upon the question 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 NeA\i;on are set forth in a number 

 of queries published in his work entitled 'Optics.' As is well known, 

 Newton believed in the material 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 



